In a large cohort of women, the risk of subarachnoid haemorrhage was significantly associated both with cigarette smoking and with use of oral contraceptives (O.C.S.). The risk of cigarette smokers was 5.7 times that of non-smokers; for current users of O.C.S. the risk was 6.5 times that of non-users. The risk in women who both smoked cigarettes and used O.C.S was 22 times that of women who neither smoked nor used O.C.S. Past use of O.C.S was also significantly associated with increased risk of subarachnoid haemorrhage, the relative risk being 5.3. There was some evidence that risk may increase with duration of O.C. use. In this series no significant associations were found between occurrence of subarachnoid haemorrhage and history of hypertension or of migraine headache.