Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is an inherited arterial disease of the brain recently mapped to chromosome 19. We studied 148 subjects belonging to seven families by magnetic resonance imaging and genetic linkage analysis. 45 family members (23 males and 22 females) were clinically affected. Frequent signs were recurrent subcortical ischaemic events (84%), progressive or stepwise subcortical dementia with pseudobulbar palsy (31%), migraine with aura (22%), and mood disorders with severe depressive episodes (20%). All symptomatic subjects had prominent signal abnormalities on MRI with hyperintense lesions on T2-weighted images in the subcortical white-matter and basal ganglia which were also present in 19 asymptomatic subjects. The age at onset of symptoms was mean 45 (SD [10-6]) years, with attacks of migraine with aura occurring earlier in life (38.1 [8.03] years) than ischaemic events (49.3 [10.7] years). The mean age at death was 64.5 (10.6) years. On the basis of MRI data, the penetrance of the disease appears complete between 30 and 40 years of age. Genetic analysis showed strong linkage to the CADASIL locus for all seven families, suggesting genetic homogeneity. CADASIL is a hereditary cause of stroke, migraine with aura, mood disorders and dementia. The diagnosis should be considered not only in patients with recurrent small subcortical infarcts leading to dementia, but also in patients with transient ischaemic attacks, migraine with aura or severe mood disturbances, whenever MRI reveals prominent signal abnormalities in the subcortical white-matter and basal ganglia. Clinical and MRI investigations of family members are then crucial for the diagnosis which can be confirmed by genetic linkage analysis. The disease is probably largely undiagnosed.