Chester v Afshar [2004] UKHL 41 is an important English tort law case regarding causation in a medical negligence context.

Miss Chester was referred to Dr Afshar, a neurological expert, about some lower back pain.

It is a basic principle of good medical practice that adults should consent on a fully informed basis to surgery, aware of all risks.

Lord Steyn argued that such a right "must be given effective protection whenever possible", warranting a "modest departure from traditional causation principles", emphasising his view by quoting Ronald Dworkin: The value of autonomy, on this view, derives from the capacity it protects: the capacity to express one's own character - values, commitments, convictions, and critical as well as experiential interests - in the life one leads.

We allow someone to choose death over radical amputation or a blood transfusion, if that is his informed wish, because we acknowledge his right to a life structured by his own values.Lord Walker finished his speech with the comment, I agree with Lord Steyn and Lord Hope that such a claimant ought not to be without a remedy, even if it involves some extension of existing principle, as in Fairchild v. Glenhaven Funeral Services Ltd [2003] 1 AC 32 (see especially the speech of my noble and learned friend Lord Bingham of Cornhill at paras 8-13).

[2]Lord Bingham felt that even though Dr Afshar had been found not to have informed Miss Chester about the 1–2% risk of surgery failure, this did not mean that causation had been shown.

But, in the ordinary run of cases, satisfying the "but for" test is a necessary if not a sufficient condition of establishing causation.