A case of myoglobinuria of idiopathic origin is described. A diagnostic triad of (1) muscular pseudoparalysis, (2) a highly colored urine containing a benzidine positive pigment, and (3) plasma free from visible discoloration was present. Identification of the urinary pigment as myoglobin was established by spectrophotometry, and the presence of accompanying metmyoglobinuria was detected. Exercise of the lower extremities was the precipitating factor during each attack of myoglobinuria. Attempts to induce attacks by means other than exercise were unsuccessful, and no amelioration of myoglobinuria could be produced by treatment with ascorbic acid or cortisone. The myoglobinopathies are discussed together with the present concept of the physiology of myoglobin. The differentiation of myoglobinurias from hemoglobinuria and porphyrias is readily accomplished by the laboratory procedures indicated above.