A 17-year-old boy had exercise-induced cramps and myoglobinuria. The mother had myalgia and weakness after exercise but the father was asymptomatic. Muscle biopsy was normal in the father but showed glycogen storage and absent or markedly decreased histochemical stain for phosphorylase in mother and son. Autosomal dominant McArdle's disease was considered likely, but biochemical studies showed that muscle phosphorylase activity was 0.6% of normal in the son, 20% in the mother, and 45% in the father, with corresponding decreases of cross-reacting material by immunotitration. These data suggest autosomal recessive transmission. One of the parents was clinically silent and the other was a manifesting heterozygote.