The magnitude of hGH reserve was investigated in young (16-40 years old) diabetic patients. Forty patients were examined with the arginine infusion test, twenty three with a sensitized version of the L-Dopa test. The patients were divided into four groups based on the type of their illness (IDDM or NIDDY) and the clinical stage of IDDM. It was concluded that the hGH reserve of the various diabetic groups differs from the hGH reserve of healthy persons as well as from one another. Taking into consideration the possible causes of these differences, it is suggested that the hGH reserve depends primarily on the metabolic condition of the patients, while the type of diabetes (IDDM or NIDDY) and the insulin therapy used may also be important contributing factors. Achievement of a good metabolic control within the same type of diabetes leads to the normalization of the previously pathologic hGH reserve.