To quantify the placebo response of prophylactic therapy in migraine, a meta-analysis of prophylactic, double-blind, placebo controlled migraine studies was performed. The total analysis included 22 studies testing 19 different products, including 2013 patients, of which 828 were treated with placebo. A reduction in migraine attacks of 50% or more (responders) was seen in 23.5%+/-8.0 (95% C.I. 18.3-28.8%) of the patients in the placebo groups and 45.5%+/-15.5 (95% C.I. 37.4-53.6%) in the active groups. A reduction in migraine attacks of 16.8%+/-12.7 (95% C.I. 10.9-22.6%) was observed in the placebo groups and 41.8%+/-11.7 (95% C.I. 36.9-46.6%) in the active groups. We propose that if the percentage of responders in an open-label prophylactic trial in migraine is above 35-40%, or if a reduction in migraine attack frequency is found of 40% or more, further studies are indicated to determine the prophylactic activity of the drug. In all studies included in this analysis, no placebo response was seen above these limits.