A retrospective epidemiological study on the first episode of stroke in young adults aged 15-44 years was carried out in the territory of the Local Health Unit No. 9 in Reggio Emilia (46,491 km2), Italy, from 1987 to 1989. 29 patients were identified: 17 were affected with cerebral infarction and 12 with hemorrhage. All young patients were discharged with diagnostic codes 430-438 according to the International Classification of Disease, i.e. the criteria of the World Health Organisation for stroke definition. All patients had computed tomography or necropsy. The average annual incidence rate per 100,000 population aged 15-44 for all strokes was 13.6 and the 95% confidence interval (CI 95%) was 9.1-19.6. The general population of the same age on January 1, 1987, was 69,845 and 71,920 on December 31, 1989; the incidence rate of stroke was 14.0 for males (CI 95% 7.9-2.3) and 13.2 (CI 95% 7.1-22.2) for females. The average annual incidence rates were 8.0 (CI 95% 4.7-12.2) for cerebral infarction (8.4, CI 95% 3.9-16 for males, 7.6, CI 95% 3.3-14.9 for females), 5.6 (CI 95% 2.9-9.9) for cerebral hemorrhage and 2.8 (CI 95% 1.0-6.1) for both subarachnoid (SAH) and intracerebral hemorrhage (ICH). Based on angiography or necropsy findings, aneurysms or arteriovenous malformations were present in 83% of the patients with SAH and in 66% of the patients with ICH. The 1-month fatality ratio was 0 for cerebral infarction, 50% for SAH and 33% for ICH.(ABSTRACT TRUNCATED AT 250 WORDS)