Pulmonary function testing of 14 male children with pseudohypertrophic muscular dystrophy revealed reduced mean percent predicted values for forced vital capacity (FVC) (67%), maximal voluntary ventilation (MVV) (67%), and peak expiratory flow rate (PEFR) (72%). Percentages for one-second forced expiratory volumes (FEV1) in relation to forced vital capacity (FEV1:FVC) were near normal. Clinical findings included limited intercostal activity, ineffective coughs, and marked weakness of abdominal and neck flexor musculature. According to obtained histories, the subjects were unexpectedly free of common respiratory illness. Interpretation of the data suggested that relationships existed between age, scoliosis, and reduced vital capacity (VC). Reduced values obtained from pulmonary testing were attributed primarily to respiratory muscle involvement, physical inactivity, and restriction of thoracic movement. It is suggested that periodic pulmonary function evaluation should become an integral part of the treatment program for patients with pseudohypertrophic muscular dystrophy.