Basal (8.00 a.m.) plasma ACTH-radioimmunoassay (ACTH-RIA) levels were studied in 32 cases of endogenous Cushing's syndrome (17 Cushing's disease, 13 adrenocortical tumors, and 2 ectopic ACTH syndrome) and 11 normal volunteers. There were overlaps in the ranges of plasma ACTH-RIA levels among patients with Cushing's disease, adrenocortical tumors, and normal volunteers but not ectopic ACTH syndrome. By using different plasma ACTH-RIA levels as cut-off points in differentiating ACTH-dependent from ACTH-independent Cushing's syndrome, the level of 30 pg/ml had the highest diagnostic efficacy with a 94.7 per cent sensitivity, a 84.6 per cent specificity and a 90.6 per cent diagnostic accuracy.