Based on ethnographic research from two distinct French settings, the author examines the lying of doctors, and the lying of patients. The first situation is that of medical practitioners, specialists in the treatment of alcoholism, who affirm to ex-drinkers that it is impossible to drink normally again after treatment, without falling back into dependency, whilst knowing of the existence of contradictory cases. The second situation is that in which a certain number of patients find themselves, and who lead their doctors to believe that they have been taking their medication and dissimulate their real behaviour, that of non-observance of prescription. The author argues that lying, in the context of secrecy, is the expression of and the indication of a power relationship. Moreover, the rationalisation that accompanies the lie does not stop it from producing effects in contradiction to its motivation, thus exposing the conflict between therapeutic logic and social logic: the paradoxical character of lying.