OBJECTIVE--To determine whether migraine is a risk factor for ischaemic stroke in young women. DESIGN--A case-control study. SETTING--Five hospitals in Paris and suburbs. SUBJECTS--72 women aged under 45 with ischaemic stroke and 173 controls randomly selected from women hospitalised in the same centres. MAIN OUTCOME MEASURES--Ischaemic stroke confirmed by cerebral computerised tomography or magnetic resonance imaging; history of headache recorded with structured interview, and diagnosis of migraine assessed by reproducibility study. RESULTS--Ischaemic stroke was strongly associated with migraine, both migraine without aura (odds ratio 3.0 (95% confidence interval 1.5 to 5.8)) and migraine with aura (odds ratio 6.2 (2.1 to 18.0)). The risk of ischaemic stroke was substantially increased for migrainous women who were using oral contraceptives (odds ratio 13.9) or who were heavy smokers (> or = 20 cigarettes/day) (odds ratio 10.2). CONCLUSIONS--These results indicate an independent association between migraine and the risk of ischaemic stroke in young women. Although the absolute risk of ischaemic stroke in young women with migraine is low, the reduction of known risk factors for stroke, in particular smoking and use of oral contraceptives, should be considered in this group.