It is the thesis of this paper that most therapeutic trials are inadequately formulated, and this from the earliest stages of their conception. Their inadequacy is basic, in that the trials may be aimed at the solution of one or other of two radically different kinds of problem; the resulting ambiguity affects the definition of the treatments, the assessment of the results, the choice of subjects and the way in which the treatments are compared. It often occurs that one type of approach is ethically less defensible than the other, or may even be ruled out altogether on ethical grounds. We postpone consideration of this aspect of the question until a later section.