A large cohort of patients with Fabry disease is being studied to determine the natural history of the disease and how this relates to the specific mutation involved and the amount of residual alpha-galactosidase A activity. To date, we have investigated the progression of cerebral lesions and stroke, as identified by magnetic resonance imaging, and renal disease. Results have shown that cerebral lesions do not appear until 23 years of age, but are present in all patients by 55 years of age. The peak onset of proteinuria occurred in the fourth decade, and the peak onset of chronic renal insufficiency and end-stage renal disease occurred in the fifth decade of life. Renal outcome was related to the type of mutation and residual enzyme activity. Data from these studies in untreated patients will be important when assessing the long-term efficacy of enzyme replacement therapy.