We describe the biochemical characterization of lymphocyte alpha-glucosidase in a 23-year-old man with intermediate clinical features between the childhood and adult forms of glycogenosis type II (Pompe's disease). Acid alpha-glucosidase activity was markedly reduced, but immunologic cross-reactive material against human liver acid alpha-glucosidase protein could be detected, and its amount was normal. In this patient, the disorder was induced by the catalytically inactive enzyme with a normal amount of enzyme protein.