AbstractTwo siblings with McArdle disease were studied biochemically and histochemically. Case 1 was a 9‐year‐old girl and case 2 was a 14‐year‐old boy. No elevation of serum lactate after exercise was noticed in both cases. PAS staining in case 1 was markedly increased, whereas in case 2 it was moderately increased. Muscle glycogen content in case 1 was 51 mg/gr muscle and in case 2 it was 38.7 mg/gr (control: 11.2 ± 1.28 mg/gr). Muscle phosphorylase activity was completely absent in case 1 and was about 50% of normal value in case 2, by both determinations of histochemical and direct biochemical methods. SDS‐polyacrylamide gel electrophoresis revealed a little amount of muscle phosphorylase protein in both cases and also no abnormal migration of residual activity of muscle phosphorylase was found on polyacrylamide electrophoresis. These data indicate that occurrence of familial variety of muscle phosphorylase in two siblings was due to difference of muscle phosphorylase activity and also different onset of disease and clinical severity among two cases are depending on the degree of residual amount of