The efficacy and tolerability of a CO(2)-extract of feverfew (MIG-99, 6.25 mg t.i.d.) for migraine prevention were investigated in a randomized, double-blind, placebo-controlled, multicentre, parallel-group study. Patients (N = 170 intention-to-treat; MIG-99, N = 89; placebo, N = 81) suffering from migraine according to International Headache Society criteria were treated for 16 weeks after a 4-week baseline period. The primary endpoint was the average number of migraine attacks per 28 days during the treatment months 2 and 3 compared with baseline. Safety parameters included adverse events, laboratory parameters, vital signs and physical examination. The migraine frequency decreased from 4.76 by 1.9 attacks per month in the MIG-99 group and by 1.3 attacks in the placebo group (P = 0.0456). Logistic regression of responder rates showed an odds ratio of 3.4 in favour of MIG-99 (P = 0.0049). Adverse events possibly related to study medication were 9/107 (8.4%) with MIG-99 and 11/108 (10.2%) with placebo (P = 0.654). MIG-99 is effective and shows a favourable benefit-risk ratio.