Three neonatal patients, one girl and two boys, presented with infantile Pompe's disease. A generalized hypotonia with decreased tendon reflexes and heart failure due to hypertrophic cardiomyopathy dominated the clinical picture in all three; these symptoms are uniformly and characteristically present. This autosomal recessive glycogen storage disease is caused by a deficiency of lysosomal alpha-glucosidase. The diagnosis, suspected on the basis of the characteristic clinical picture and the results of simple laboratory tests, is made by measurement of the enzymatic activity or DNA analysis. Most patients die in their first year of life, no treatment being available.