Ten women with histories of migraine attacks and with diagnosis of classic or common migraineconcluded a double-blind, placebo controlled, cross-over study to assess the effectiveness of syntheticsalmon calcitonin 100 MRC U. i.m. a day in preventing migraine attacks. After a two-month run-in phase thewomen were randomly allocated to one of the following two-month sequences: placebo-calcitonin orcalcitonin-placebo. The patients filled in every day a standard card with number, duration and severity ofattacks. The mean migraine frequency per month on calcitonin was 3.7±0.7 (s.e.m.) and on placebo 7.7±1.3(r<0.01). The ratio of total score of headache intensity to the number of days of observation on calcitoninwas 0.18±0.03 and on placebo was 0.30±0.04 (r<0.01). It is concluded that calcitonin is effective forprophylactic treatment of migraine and should be useful when other first-choice treatments arecontraindicated.