Myophosphorylase deficiency: an unusually severe form with myoglobinuria.

Myophosphorylase deficiency (McArdle disease) is characterized by exercise intolerance that usually starts in childhood. Severe cramps and myoglobinuria are rarely problems in children. We describe an 8-year-old boy with exercise-induced myoglobinuria; he was homozygous for the mutation most commonly encountered in patients with typical McArdle disease.