He is credited with the classification of blood into four types (I, II, III, IV).

During World War I, Janský served two years as a doctor at the front until a heart attack disabled him.

After the war he worked as a neuropsychiatrist in a military Hospital (Vojenská nemocnice).

He had angina pectoralis and died of ischaemic heart disease in Dolní Mokropsy on 8 September 1921, at the age of 48.

(At the time, Janský was unaware of the work of Karl Landsteiner, whose discovery of the A, B, and O blood types earned him the Nobel Prize in Physiology or Medicine in 1930.)