The lysosomal storage disorder glycogenosis type II, caused by a deficiency of lysosomal alpha-glucosidase, is very heterogeneous in its clinical presentation. It has been suggested that this heterogeneity may be due to differential expression of neutral alpha-glucosidases. We have therefore analysed the activity of the major neutral alpha-glucosidases in cultured fibroblasts or muscle cells from 26 patients with glycogenosis type II. The results indicate that there is no correlation between the expression of neutral alpha-glucosidase isoenzymes and the clinical phenotype of this disease.