A population-based study specifically addressing stroke in young adults (aged 15-44 years) was conducted in Florence, Italy, from 1983 to 1985. We identified 47 cases of first stroke by means of a daily check of the medical facilities of the city and nearby towns and a review of death certificates. Patients were assessed by a neurologist shortly after the onset of the stroke, and computed tomography or autopsy was performed in 96%. The average annual incidence rate for all stroke (cases per 100,000 population per year) was 9.0 (95% confidence interval 5.8-13.4) for males and 8.7 (95% confidence interval 5.5-13.0) for females. The average annual incidence rates for the pathologic types of stroke were 3.4 for cerebral infarction, 3.2 for subarachnoid hemorrhage, and 1.9 for intracerebral hemorrhage. The case-fatality ratio was 23.4% at 1 month. Among patients with ischemic strokes, atherosclerosis and cardiac disease accounted for 50% of the cases. Based on angiography or autopsy findings, aneurysm or arteriovenous malformation were demonstrated in 88% of the patients with subarachnoid hemorrhage. In 50% of the patients with intracerebral hemorrhage, no cause of bleeding was detected. Our study may supply information about stroke pathologic types in an unselected series of young adults.