A 2-year-old boy became ill with diabetic ketoacidosis complicated by severe rhabdomyolysis. He completely recovered from the rhabdomyolysis, but has persistent insulin-dependent diabetes mellitus (IDDM). Serological studies showed that the patient's serum contained high titers of coxsackievirus B4 antibody, suggesting that the development of rhabdomyolysis and IDDM may have been related to this infection. A review of the records of 133 patients admitted with onset of IDDM disclosed one additional patient with marked myoglobinuria, and 11 patients with orthotolidine-positive urine in the absence of hematuria. These findings suggest that myoglobinuria may not be uncommon at the onset of IDDM.