Serum growth hormone values in 37 patients with diabetic ketoacidosis were 5.4±0.8 ng/ml (S.E.M.) in males and 6.7±1.1 ng/ml in females before treatment; while in five hyperosmolar non-ketotic patients the HGH concentration was 3.9±0.5 ng/ml. One hour after insulin 90% of patients showed a rise in HGH, to a mean of 33.7±9.8 ng/ml for males and 25.5±6.0 ng/ml for females in ketoacidosis; and to 27.1±9.9 ng/ml for hyperosmolar coma patients. The rise, which was transient, was inversely correlated with pretreatment plasma glucose, the l h plasma glucose concentration and plasma urea, and directly proportional to the % fall in blood glucose after 1 h. When the ketoacidosis patients were divided into two groups according to HGH response, those with a small response had the greater disturbances of plasma glucose, blood ketone bodies, blood lactate, plasma urea, blood pH, and blood pressure, the smaller 1 h fall in blood glucose, and the higher mortality. Thus the most severely ketoacidotic patients had the poorest growth hormone response. Growth hormone is probably of little importance as an insulin antagonist in diabetic coma.