Proliferative myositis presented in a 7-month-old female infant as a poorly circumscribed mass growing rapidly within the interscapular skeletal muscle. Muscle fascicles and individual fibers were dissected by an inflammatory infiltrate of lymphocytes, plasma cells, and histiocytes immersed in a basophilic intercellular substance. Numerous ganglionlike giant cells were present and had ultrastructural features suggestive of histiocytic origin. The possible relation to other rapidly growing lesions that appear in soft tissues is discussed. This probably represents the first reported case of proliferative myositis in a pediatric patient.