Female heterozygous patients with Fabry disease are difficult to identify because of the relatively high residual activity of alpha-galactosidase. We systematically evaluated the activities of various lysosomal enzymes in dried blood samples from Fabry patients and found that the beta-glucuronidase activity was frequently elevated. The ratio of alpha-galactosidase to beta-glucuronidase proved to be a helpful tool for the diagnosis of female Fabry disease patients.