Ten male patients and 6 female patients with newly diagnosed insulin dependent diabetes mellitus and significant ketosis were studied before and during the first year of insulin treatment. At onset plasma concentrations of testosterone and androstenedione were significantly (P less than 0.02) decreased in female patients when compared to healthy sex- and age-matched controls. Plasma concentrations of testosterone and androstenedione increased significantly (P less than 0.001 and P less than 0.05) in both sexes during insulin therapy. Plasma concentrations of testosterone were not significantly different from controls in both sexes after 4 days of insulin treatment and remained so during the first year of insulin treatment. Plasma concentrations of androstendeione were not significantly different from controls in both sexes after 4 days of insulin treatment, but increased to levels significantly raised in both males (P less than 0.001) and females (P less than 0.05) after 7 days and during the first year of insulin treatment.