When are placebo-controlled trials no longer appropriate?

This paper presents a standard for assessing the validity of placebo-controlled trials in circumstances in which such trials might be unjustly denying appropriate therapies to members of the control group. This standard categorizes the types of risks that can or cannot be imposed upon consenting research subjects in such control groups. The paper also shows how needed research can be conducted while respecting the proposed ethical standard. Both the problem and the proposed standard are illustrated by reference to the major trials of the thrombolytic agents.