The aim of the present study was to assess the efficacy and tolerability of single oral doses of 50 mg and 100 mg of diclofenac-K compared to placebo in migraine sufferers during three attacks. The study was conducted in a double-blind, randomized, placebo-controlled, three-period, within-patient comparative trial; 72 migraine patients were treated with diclofenac-K (50 mg or 100 mg) or placebo at six centres (1 in Sweden and 5 in Finland). The primary efficacy end-point was the change in pain intensity assessed on a 100 mm Visual Analogue Scale (VAS) at 120 min after taking the study medication. We found that 50 mg and 100 mg of diclofenac-K reduced the pain intensity significantly better than placebo (p = 0.003 and p = 0.001, respectively), without difference between the doses; 100 mg diclofenac-K was significantly better than placebo in improving phonophobia, photophobia, working ability and need for rescue medication. Diclofenac-K 50 mg or 100 mg is an effective and well-tolerated acute treatment for migraine headache and its associated symptoms. The higher dose of diclofenac-K was only marginally more effective than the lower dose.