The frequency of nontraumatic rhabdomyolysis in diabetic ketoacidosis was investigated by serial measurements of the serum levels of myoglobin and the serum activity of creatine kinase isoenzyme MM in 12 consecutively admitted ketoacidotic patients. In 5 patients (Group 1) we found hypermyoglobinaemia and elevated activity of creatine kinase isoenzyme MM on admission to hospital, whereas these two variables were normal in 7 patients (Group 2). On admission significantly higher median blood glucose levels and higher median serum osmolality were found in Group 1 than in Group 2 (for blood glucose: 49.6 mmol/l versus 19.0 mmol/l, p less than 0.02; for serum osmolality: 360 mosm/kg H2O versus 315 mosm/kg H2O, p less than 0.05). Decreased renal function was found in Group 1 as reflected by significantly higher beta 2-microglobulin serum concentrations in Group 1 compared with Group 2 on admission (median values 4.1 mg/l versus 1.7 mg/l, p less than 0.01) and during the first 3 days of therapy. The serum concentration of hypoxanthine (an indicator of the cellular energy state) was elevated in all patients on admission, with no difference between patients with or without hypermyoglobinaemia. In conclusion, our findings suggest that nontraumatic rhabdomyolysis with hypermyoglobinaemia and elevated serum activity of creatine kinase isoenzyme MM may be a hitherto unrecognized common feature of diabetic ketoacidosis.