McArdle disease classically presents in childhood or adolescence. Rarely does it become symptomatic for the first time in late adulthood, with the onset of progressive muscle wasting and weakness. Our patient is unusual in that despite a life of physical vigor, she developed immobilizing cramps, stiffness, and muscle swelling abruptly at age 60. She had no previous symptoms of muscle disease. The diagnosis was indicated by the ischemic forearm test, which produced muscle contracture and no rise in venous lactate levels, and confirmed by histochemical, electrophoretic, and biochemical studies that showed complete absence of myophosphorylase. This case defines a new variant of the late-onset type and raises important questions about compensatory mechanisms, inheritance patterns, and etiological factors in myophosphorylase deficiency.