Selective adrenal vein catheterization was done preoperatively in 11 patients with Cushing's syndrome owing to either an adrenal adenoma (6) or an adrenal carcinoma (5). Peripheral and adrenal venous blood (tumor side) was analyzed for progesterone, 17-hydroxyprogesterone, testosterone, delta-4-androstenedione, dehydroepiandrosterone, dehydroepiandrosterone sulfate, cortisol and 11-deoxycortisol. Comparisons were made of the ratios of precursors to products in the androgen and cortisol production pathways. In adrenal vein samples, cortisol levels were higher in the adenoma patients (p equals 0.06). These patients had a higher ratio of 17-hydroxyprogesterone to delta-4-androstenedione (p less than 0.04). The ratio of 11-deoxycortisol to cortisol was higher in the carcinoma patients (p less than 0.02). In the peripheral samples, the carcinoma patients had higher levels of testosterone and 11-deoxycortisol (p less than 0.02) and progesterone (p equals 0.05). The most striking differences were in the peripheral levels of 17-hydroxyprogesterone, delta-4-androstenedione and dehydroepiandrosterone, which were 10 to 20 times higher in the carcinoma group (p less than 0.005). The ratios of progesterone to 17-hydroxyprogesterone and 17-hydroxyprogesterone to 11-deoxycortisol were higher in the adenoma group (p less than 0.005 and p equals 0.06, respectively). The peripheral blood ratio of 11-deoxycortisol to cortisol was higher in the carcinoma group (p less than 0.03). These data indicate that peripheral and adrenal venous steroid levels and ratios of precursors to products are significantly different in the 2 groups. The carcinomas have a block in the conversion of 11-deoxycortisol to cortisol with a subsequent accumulation in androgen and cortisol precursors. The elevations in 17-hydroxyprogesterone, delta-4-androstenedione and dehydroepiandrosterone, and in the ratio of 11-deoxycortisol to cortisol in the carcinoma group are most striking in the periphery. Peripheral blood steroid measurement in patients with Cushing's syndrome and an adrenal mass aids in the preoperative differentation of carcinoma from adenoma.