X-linked muscular dystrophy has been separated into two types that are generally considered to be distinct entities. We have investigated three families with X-linked muscular dystrophy who demonstrate remarkable intrafamilial variability. In one family 2 brothers with a benign type had a maternal uncle who was affected with the Duchenne type. In another family, 4 members had the benign type but a fifth was much more severely affected than in the classic Duchenne dystrophy; therefore the presence of an "aggressive form" is proposed. A third family also had both benign and severe types. A search of the literature revealed families in which the severe and benign types coexisted. The genetic determinants of this heterogeneity are not yet known. Clinical similarities between benign and severe types of X-linked muscular dystrophy and the presence of families with both types suggest that the two are intimately related.