Out of 13 patients with congenital muscular dystrophy myoglobinemia was proven in four, and nine of 13 cases of Duchenne dystrophy showed myoglobinemia by counterimmunoelectrophoretic technique. Serum myoglobin was positive in 14 out of 15 patients whose serum creatinine phosphokinase (CPK) levels rose above 2500 units, whereas positive myoglobinemia was obtained in only one patient in eleven with lower CPK levels. All the myoglobinemic patients were less than 5 years of age in the case of congenital muscular dystrophy and less than 9 years of age in the case of Duchenne dystrophy. Thus, the leakage of myoglobin into the serum from the damaged muscle seemed closely correlated to the age of the patients and level of serum CPK, but not to the type of disease.