Migraine is a cyclical condition in which the cycles may occur or remit in an unpredictable fashion. The goal of prophylactic antimigraine therapy is to induce remission of an active cycle of migraine or to inhibit occurrence of individual headaches until the cycle remits for other reasons. The placebo effect probably takes advantage of these cycles. The placebo effect is significant and quite potent. The placebo effect may be divided into (1) the initial effect and (2) the continuing effect which is seen following a stabilization period. The initial placebo effect is dramatic with 62% of 188 subjects improving by 75% after 4 weeks of placebo. The continuing effect is demonstrated by occurrence of 75% further improvement in 28% of 282 subjects in 7 studies in which comparison of results after 4–12 weeks was made with a placebo stabilization period. The placebo effect is a very potent one and must be taken into account in designing and carrying out studies of migraine therapy.