Over sixty living members of a four-generation kindred underwent complete medical examinations with appropriate laboratory tests to evaluate the presence of two autosomal dominant conditions: an unusual form of muscular dystrophy and the Pelger-Huet anomaly of the leukocytes. The study started with the discovery of a person who manifested both genetic conditions. Clinically, the muscular dystrophy was characterized by gradual onset and slow progression of motor weakness predominantly in the proximal limb muscles with consistent sparing of the face. Analysis of critical matings suggests that the gene loci controlling these two autosomal dominant conditions may be linked with a recombination fraction of approximately 0.25. Since the muscular dystrophy does not become obvious until the third to fourth decade, whereas the Pelger-Huet anomaly is evident at birth, the latter provides a possible marker for studies of the muscular disorder in its preclinical stages. Muscle biopsy specimens taken from affected adults showed typical abnormalities by light microscopy. Electron microscopic studies of one of these specimens showed destruction and degeneration of myofibrils. Similar investigations of children, some of whom are presumed to carry the gene for muscular dystrophy, showed no alteration unequivocally indicative of disease.