Introduction: Anecdotal reports suggest that clopidogrel may prevent migraine attacks. We undertook a pilot randomised trial. Method: We randomised consecutive migraineurs with four to 15 headache days per 28-day month to receive clopidogrel 75 mg or placebo daily for three months. Headache was primarily assessed with a headache diary. Results: There were no statistically significant treatment effects. The number of headache days fell by 1.9 on clopidogrel and 1.6 on placebo (adjusted difference 0.02, CI-2.07 to 2.12). Headache severity rose by 0.14 points (out of 10) on clopidogrel, and fell by 0.63 on placebo; treatment effect 0.7 points (CI-0.11 to 1.57). The main treatment effect did not depend on the presence or absence of migraine with aura at baseline, a patent foramen ovale (PFO) or atrial septal aneurysm. Discussion: The evidence is inconclusive, but a multicentre trial would be feasible recruiting from primary care.