We studied a 21-year-old patient with clinical, biochemical and histochemical evidence of myophosphorylase deficiency and unusual repetitive episodes of pigmenturia. His muscle biopsy also revealed morphological signs of mitochondrial proliferation and a defect of complex I of the respiratory chain. His mother had exercise intolerance without myoglobinuria and no histochemical evidence of myophosphorylase deficiency. In muscle, the mother showed some ragged-red fibers, normal respiratory chain levels and a significant residual phosphorylase activity. Molecular genetic analysis revealed that the proband was homozygous for the mutation commonly found in McArdle's disease. The mother, father, and the five siblings were all heterozygous for the same mutation. Mitochondrial DNA analysis of the proband's muscle failed to demonstrate known mutations associated with his clinical pattern. Moreover, we sequenced his tRNA(Leu(UUR)) gene, a hot spot for mutations, showing no abnormality.