Because the cause and prognosis of ischemic stroke in adults younger than 30 years of age are not known, we observed 41 such patients (26 females and 15 males) using a standard protocol of investigations, including cerebral angiography and echocardiography. Mitral valve prolapse and arterial dissection accounted for 51% of infarcts, migrainous infarction was the likely cause in 15% of infarcts, and uncommon causes accounted for 34% of infarcts. Atherosclerosis played a role in only two patients. Two thirds of the women were taking oral contraceptives, which may have been the primary cause of stroke in one woman who had recurrent venous thromboses followed by thrombosis of the aortic arch. Three patients (7.3%) died of acute causes. During follow-up (mean, 46 months), the annual incidence of death (0.7%) and recurrent stroke (0.7%) was low. Eighty-one percent of the survivors could return to work. We conclude that cerebral angiography and echocardiography are indicated in all adults younger than 30 years of age. After the acute phase of stroke, prognosis is reasonable.