18-Hydroxycortisol and 18-oxocortisol have been isolated from the urine of patients with aldosterone producing adrenocortical adenomas, but not from those with idiopathic hyperaldosteronism associated with bilateral adrenal hyperplasia. These C-18 oxygenated cortisols are biosynthesized by the substitution of cortisol for the normal substrate, corticosterone, in the terminal oxidase system required for the biosynthesis of 18-hydroxycorticosterone and aldosterone. To make use of this biochemical difference between the two groups in the preoperative diagnosis of primary aldosteronism, we have developed and utilized a specific primary standard analytical method, stable isotope dilution mass fragmentography, for quantifying 18-hydroxycortisol and the tetrahydro metabolite of 18-oxocortisol in 24-h urine samples. The normal range by this technique of 4.6 +/- 1.8 micrograms/day tetrahydro 18-oxocortisol and 43 +/- 23 micrograms/day 18-hydroxycortisol in urine was lower and narrower than previous estimates using other methods. Excretion of the 18-oxocortisol metabolite ranged from 2-12 micrograms/day in bilateral hyperplasia and 17-1203 micrograms/day in typical adenomas. 18-Hydroxycortisol excretion similarly separated bilateral hyperplasia (23-59 micrograms/day) from typical adenomas (60-2750 micrograms/day). The cortisol C-18 oxidation pathway describes a unique steroidogenic mechanism in the aldosteronoma not present in idiopathic aldosteronism due to bilateral adrenal hyperplasia and as such provides a basis for the biochemical classification of primary aldosteronism and the differentiation of these two groups. This unique biochemistry was also observed in unilateral hyperplasia but not in the renin-dependent aldosteronoma.