A unique family is presented which consists of a patient with the juvenile muscular dystrophy form of glycogenosis type II and four healthy individuals, both parents and sisters, with low acid alpha-glucosidase activity. It was almost impossible to distinguish the homozygote from the heterozygous members by lymphocyte assay alone. In cultured skin fibroblasts, acid alpha-glucosidase activity measured with a synthetic substrate was less than 1% of the normal mean value in the patient and about 15% in the parents. The activity toward glycogen was not detectable in the patient and was about 30% of the normal mean value in the parents. These values are also lower than expected in heterozygotes. To explain these results properly, a new mutant allele of acid alpha-glucosidase is proposed. Both parents could be compound heterozygotes for the pseudodeficiency allele and the juvenile form of glycogenosis type II allele.