A systematic investigation of neoplastic infiltration in skeletal muscle was performed utilizing up to 12 selected muscle samples in 82 autopsies of patients with leukemia or lymphoma. Tumor foci were recorded in one or more muscles in 43 of these 82 cases. The most extensive involvement was found in monocytic leukemia, where 36% (48 out of 132 muscle samples taken) were infiltrated. There was general correlation between muscular involvement, degree of visceral infiltration, and, in leukemia, also the number of circulating leukemic cells. Secondary changes in skeletal muscle occurring independently of leukemic-lymphomatous infiltration or compromise of neurovascular supply are alluded to. The study showed that, contrary to common assumption and scant literature reports, skeletal muscle involvement in leukemia-lymphoma is not a rare phenomenon.