A basal plasma Cortisol value taken in a physically unstressed state in 68 patients with or without hypothalamic-pituitary-adrenocortical disease was compared with the maximal plasma Cortisol concentration during an insulin tolerance test. There was a strong positive correlation between the values. Basal Cortisol levels above 300 nmol/1 (RIA method) almost excluded ACTH-cortisol insufficiency and those below 100 nmol/1 strongly suggested dysfunction. A repeated basal Cortisol estimation within a month was especially valuable in categorizing patients with levels between 100 and 200 nmol/1. We suggest that a basal Cortisol measurement may be used as a first laboratory test in patients evaluated for possible hypothalamic-pituitary-adrenocortical insufficiency; in many patients, this approach obviates more sophisticated and expensive testing.