Nemaline myopathy occurred sporadically in a 59-year-old woman. She had slowly progressive weakness. A muscle biopsy specimen showed nemaline rods, increased variation in fiber size, apparent loss of Type IIb fiber differentiation, and a "moth-eaten" pattern of the intermyofibrillar network. At least 44 cases of this disorder have been reported. Clinically, they showed several different modes of onset, patterns of muscle group involvement, and rates of progression. There was a suggestion of familial involvement in at least half. In addition to the presence of nemaline rods, pathological abnormalities of muscle biopsy specimens were quite variable. Results of recent biochemical studies have shown abnormal myosin in a patient with rod myopathy. A hypothesis is proposed that relates the diverse clinical and histological features of this disease to a hereditary molecular abnormality of myosin synthesis.