While Casualty was launched on 6 September 1986, and its spin-off Holby City was first aired on 12 January 1999, the first full crossover episode between the two programmes was not broadcast until 26 December 2004.

The station's Controller of Drama approved of the idea and had the crossover commissioned, spearheaded by Casualty's executive producer Mervyn Watson, and Holby City's Tony McHale.

Once regular production began again, the availability of cast members set to appear in the crossover was limited, and both series had to rely for the most part on the remaining characters who were not in the special.

Additional filming took place at the Holby City set in Elstree, as well as the Queen Elizabeth Hospital for Children, a Barratts office block in Brentford and an ex-MOD tank testing site in Chobham.

An interactive episode of Casualty@Holby City was one of the headlining shows of the season, allowing viewers to vote by phone to determine the outcome of a fictional organ donation.

98,800 viewers voted, with a 65 percent majority favouring that the organ be received by Lucy, a young cystic fibrosis patient, over Tony, a widow twenty years her senior.

Rather than dividing the episodes between the two series' crews as had previously been standard, this crossover operated as an entirely separate production, with Diana Kyle producing and Paul Harrison directing.

The first crossover, broadcast in December 2004, features Casualty's emergency medicine consultant Harry Harper (Simon MacCorkindale), consultant paediatrician Jim Brodie (Maxwell Caulfield), Clinical Nurse Manager Tess Bateman (Suzanne Packer), paramedics Luke Warren (Matthew Wait) and Josh Griffiths (Ian Bleasdale) and receptionist Bex Reynolds (Sarah Manners).

Additional Casualty characters who appear are paramedic Comfort Jones (Martina Laird), senior staff nurse Abs Denham (James Redmond), Emergency Medical Technician Nina Farr (Rebekah Gibbs) and specialist registrar Maggie Coldwell (Susan Cookson).

Holby City's critical care consultant Lola Griffin (Sharon D. Clarke) and matron Lisa Fox (Luisa Bradshaw-White) also star.

Holby City characters who appear are registrar Diane Lloyd (Patricia Potter), matron Chrissie Williams (Tina Hobley), consultant obstetrician Owen Davis (Mark Moraghan), medical student Matt Parker (Adam Best), staff nurse Tricia Williams (Sharon Maughan) and cardiothoracic consultant Ed Loftwood (Graeme Garden).

[13] "Deny Thy Father", the two-part December 2005 crossover, stars Casualty's MacCorkindale, Bleasdale, Packer, Wait, Cookson, Thorp, Mellor and Carling, as well as Clinical Nurse Specialist Charlie Fairhead (Derek Thompson) and receptionist Sam Bateman (Luke Bailey).

Mealing, Quarshie, Jacobs and Clarke of the Holby City cast also star, along with general surgical registrar Nick Jordan (Michael French) and midwife Mickie Hendrie (Kelly Adams).

Freema Agyeman plays Kate Hindley, a patient trapped under rubble following the collision, and Keith-Lee Castle appears as isolation ward doctor Phil.

Steven Pinder plays Tony Harvey, a potential recipient of Matt's donated heart, and Nina Wadia appears as his girlfriend Jean.

Ade Sapara appears as Edgar Muzenda, a poet and political refugee involved in a car accident, and Nikki Amuka-Bird plays his pregnant wife Moji.

Three patients are introduced: Matt, a thirty-year-old man involved in a road traffic accident, who arrives at Holby General having suffered significant head trauma and is later pronounced brain-stem dead; Lucy, a nineteen-year-old student who has cystic fibrosis and is awaiting a heart and lung transplant, and Tony, a forty-one-year-old ex-smoker in the end stages of heart failure, who is also in need of a transplant.

While Matt's parents and pregnant wife deliberate over the decision to donate his organs, Lucy's father and Tony's fiance create conflict amongst the staff over which patient is more deserving of a transplant.

Consultants Ric Griffin and Harry Harper interview candidates for a locum general surgery position, including former staff member Nick Jordan.

[29] Thomas Sutcliffe of The Independent called the crossover "the funniest programme on television", with a "sublimely cloth-eared script", describing it as: "A masterpiece of sustained comic timing [which] had this casual viewer of the dramas that spawned it scrabbling wildly for an explanatory hypothesis.

"[30] The interactive episode was deemed "a horrible mess" by Shelley,[31] while Garry Bushell of The People kept his criticism brief, limiting his review to: "Casualty@Holby City?

"[39] The Daily Post were more positive, stating that the episodes "will have Casualty and Holby City fans glued to their TV sets over the festive period.

"[9] In November 2007, Kyle re-iterated that the separate series' 52-week filming schedules make it logistically difficult to produce crossovers, but revealed that the production teams were trying to create more opportunities for the two shows to merge.

Another smaller scale crossover event was broadcast in August 2016, in "Too Old for This Shift" and "Protect and Serve" The episodes were co-written by Matthew Barry and Andy Bayliss (Casualty) and Steve Brett and Joe Ainsworth (Holby City).

The episodes follow on from the cliffhanger at the end of the thirtieth series, by focusing on the events surrounding the aftermath of an accident involving Connie Beauchamp (Amanda Mealing) and her daughter Grace (Emily Carey), including a helicopter crash outside Holby City Hospital's emergency department, during celebrations of Charlie Fairhead's (Derek Thompson) thirtieth year working at the hospital.

Consultants Connie Beauchamp (Amanda Mealing) and Jac Naylor (Rosie Marcel) also come into conflict when they learn there is only one theatre available while treating two of their colleagues.