AbstractA slowly progressive autosomal dominant neuromuscular disease–termedspheroid body myopathy–is described in four successive generations and documented by muscle biopsies in five patients of two generations. With an onset in adolescence, the disease proceeds to some motor incapacitation, but life span is apparently not shortened. The salient morphologic feature is the presence of spheroid bodies, chiefly occurring in type 1 myofibers. Ultrastructurally, these spheroid bodies are composed of fine filaments but are devoid of organelles; in some cases they resemble cytoplasmic bodies. “Smearing in the l‐band” is a frequent and early finding. At a later age, signs of denervation are also present, both clinically and in muscle biopsies. The clinical and morphologic features justify the designation of this neuromuscular condition as a distin