The therapeutic effect of psychological origin of a medical treatment, said ‘‘placebo effect’’, is a fact scientifically demonstrated to uncountable occasions by all the studies that use ‘‘verum versus placebo’’. Nevertheless, it remains very badly understood, and the interest of the psychology on this subject remains even today limited. The authors propose some psychoanalytic hypothesis to highlight the unconscious relational processes, included in the relation of the care between the doctor and his patient, which encourages the placebo phenomenon. They particularly insist on the diverse modalities of the passivity in the transference, in which echoed one counter-transference marked by the ambivalence.