Elevated levels of serum myoglobin (MGB) were found in patients with several types of myopathic disorders, namely, Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), facioscapulohumeral dystrophy, and limb-girdle dystrophy. The frequency of hypermyoglobinemia was greatest in patients with DMD and BMD (91.7% and 100%, respectively). There was no precise direct relationship between serum creatine kinase (CK) and MGB levels, although the disorders with the highest CK levels tended to be the ones with the highest MGB levels. Among the patients with DMD, hypermyoglobinemia was slightly less marked in older children with restricted ability to walk than in the younger ones. Hypermyoglobinemia las noted among 43.8% of the mothers and 41.7% of the sisters of patients with DMD, slightly greater frequencies than for abnormal CK determinations in this group. Multiple serum samples were analyzed from 10 of these women, and the overall frequency of abnormal MGB levels was the same as that of abnormal CK levels; however, levels of CK and MGB were discrepant in some individual serum samples. For this reason, serum MGB determination may be a useful adjunct in the study of possible DMD carriers.