• We describe two families with a father and son with the phenotypic appearance of benign (Becker's) muscular dystrophy. Other family members were normal, though in one kindred the paternal grandfather probably had the same disorder of muscle. Muscle histology resembled that seen in Becker's muscular dystrophy with, in addition, central cores and internalized capillaries in type I fibers. These capillaries seemed to be due to an unexplained ingrowth from vessels normally located outside muscle fibers and could not be ascribed to the phenomenon of fiber splitting. The internalized capillaries were histologically normal. They may be a histologic marker for dominantly inherited pseudohypertrophic muscular dystrophy, since they have not been described in other dystrophies. We suggest that fathers and paternal relatives be evaluated, in addition to the customary screening of female family members, in all instances of apparently benign (Becker's) pseudohypertrophic muscular dystrophy.