Ten extensive families with benign (Becker type) X-linked muscular dystrophy have been studied, in which there was a total of 67 affected males. Reliable information was available on 41 of these males, 29 of whom were alive at the time of the study. The disorder was characterised by a predominantly proximal myopathy, associated with pseudohypertrophy of the calf muscles, particularly in the early stages of the disease. The early development of contractures was not a feature of the disease in these families and cardiac involvement, when present, was a late manifestation. The clinical findings in these patients are discussed in detail and compared with those in a large unselected group of patients with Duchenne muscular dystrophy. The results indicate that the best criterion for distinguishing between these two disorders is the age of becoming chair-bound. Electrocardiographic studies revealed no consistent abnormality and no evidence of an abnormal algebraic sum of the R and S waves in lead V1, as is found in patients with Duchenne muscular dystrophy. Serum creatine kinase levels are significantly elevated, particularly in the early stages of the disease, and in this way preclinical cases may be identified.