Nocturnal hypoglycemia in insulin-treated diabetic persons is often difficult to recognize clinically. It has been suggested that a useful biochemical test to demonstrate this would be the increased excretion of cortisol in the urine during the overnight period. However, of six diabetic persons who had nocturnal hypoglycemia (less than or equal to 2.5 mmol/L), plasma cortisol profiles and overnight urinary cortisol-creatinine ratios were abnormal in only one. In four others the plasma cortisol levels and cortisol excretion indices were indistinguishable from either a normal control group or a group of five diabetic subjects who did not develop nocturnal hypoglycemia. The remaining patient had a raised urinary cortisol-creatinine ratio, but did not show increased plasma levels of cortisol, growth hormone, or glucagon during the hypoglycemic phase. These data do not support the usefulness of the urinary cortisol-creatinine index as a marker of nocturnal hypoglycemia in diabetes.