Background: Strokes are an important cause of morbidity and mortality in young adults. However, in most cases the cause of the stroke remains unclear. Anderson-Fabry disease is an X-linked recessive lysosomal storage disease resulting from deficient α-galactosidase and causes an endothelial vasculopathy followed by cerebral ischaemia. To determine the importance of Fabry disease in young people with stroke, we measured the frequency of unrecognised Fabry disease in a cohort of acute stroke patients. Methods: Between February, 2001, and December, 2004, 721 German adults aged 18 to 55 years suffering from acute cryptogenic stroke were screened for Fabry disease. The plasma α-galactosidase activity in men was measured followed by sequencing of the entire α-GAL gene in those with low enzyme activity. By contrast, the entire α-GAL gene was genetically screened for mutations in women even if enzyme activity was normal. Findings: 21 of 432 (4.9%) male stroke patients and seven of 289 (2.4%) women had a biologically significant mutation within the α-GAL gene. The mean age at onset of symptomatic cerebrovascular disease was 38.4 years (SD 13.0) in the male stroke patients and 40.3 years (13.1) in the female group. The higher frequency of infarctions in the vertebrobasilar area correlated with more pronounced changes in the vertebrobasilar vessels like dolichoectatic pathology (42.9%vs 6.8%). Interpretation: We have shown a high frequency of Fabry disease in a cohort of patients with cryptogenic stroke, which corresponds to about 1.2%in young stroke patients. Fabry disease must be considered in all cases of unexplained stroke in young patients, especially in those with the combination of infarction in the vertebrobasilar artery system and proteinuria. Background: Strokes are an important cause of morbidity and mortality in young adults. However, in most cases the cause of the stroke remains unclear. Anderson-Fabry disease is an X-linked recessive lysosomal storage disease resulting from deficient α-galactosidase and causes an To determine the importance of Fabry disease in young people with stroke, we measured the frequency of unrecognized Fabry disease in a cohort of acute stroke patients. Methods: Between February, 2001, and December, 2004, 721 German adults aged 18 to 55 years suffering from acute cryptogenic stroke were screened for Fabry disease. The plasma α-galactosidase activity in men was measured followed by sequencing of the entire α-GAL gene in those with low enzyme activity. By contrast, the entire α -GAL gene was genetically screened for mutations in women even if enzyme activity was normal. Findings: 21 of 432 (4.9%) male stroke patients and seven of 289 (2.4 %) women had a biologically significant mutation within the α-GAL gene. The mean age at onset of symptomatic cerebrovascular disease was 38.4 years (SD 13.0) in the male stroke patients and 40.3 years (13.1) in the female group. The higher frequency of infarctions in the vertebrobasilar area correlated with more pronounced changes in the vertebrobasilar vessels like dolichoectatic pathology (42.9% vs 6.8%). Interpretation: We have shown a high frequency of Fabry disease in a cohort of patients with cryptogenic stroke, which corresponds to about 1.2% in young stroke patients. Fabry disease must be considered in all cases of unexplained stroke in young patients, especially in those with the combination of infarction in the vertebrobasilar artery system and proteinuria.