Eighteen patients with alpha-glucosidase deficiency have been diagnosed in Israel during the last 15 years. All patients were Palestinian Arabs, with the exception of two siblings from a Jewish Iraqi family. Clinically all patients had the infantile type (Pompe's disease), except one who had the juvenile type. Muscle glycogen content varied from 4 to 17% wet weight. Muscle alpha-glucosidase activity was zero in 10 of 17 patients examined. Among the seven patients in whom residual activity was present, the highest value was 18% of normal. Leukocyte alpha-glucosidase activity was highly variable, making this tissue unfit for enzymatic diagnosis of the disease. A marked heterogeneity was found in pH profiles of muscle and leukocyte alpha-glucosidase activity. A high prevalence of the disease in the Arab population was noted. In spite of a high rate of consanguinity, only a small number of autosomal recessively inherited diseases have been shown to be unusually prevalent in the Arab population. In view of the serious prognosis of this disease, prenatal diagnosis should be offered to affected families.