In search for a biochemical marker to differentiate between adrenocortical carcinoma (AC) and ade-noma (AA), plasma levels of the following steroids were studied preoperatively and postoperatively: 1 I-deoxycorticosterone (DOC), corticosterone (B), 11-deoxycortisol (S), and cortisol (F). Levels were measured by Sephadex LH-20 chromatography and specific radioimmunoassays. The subjects included eight children ages 2 years, 5 months to 9 years, 10 months. There were three girls and 5 boys with pseudoprecocious puberty due to adrenocortical tumors (histologically, four were AC and four, AA). The preoperative showed that DOC and S levels were elevated in all patients, F levels were elevated in four of eight children when compared with age-matched controls, whereas B was normal. Postoperatively, all levels returned to normal. The ratios of B/DOC and F/S as an index of adrenal I 1 beta-hydroxylase activity were calculated. The preoperative ratios of B/DOC were markedly decreased in all patients with AC compared to controls (7.7,4.1,5.9,1.9 versus 23.5, median), but normal in three of four patients with AA (16.2,29.6, 16.1). The F/S ratios were significantly lower in AC and AA when compared with controls. The data indicate a deficiency in 1 I beta-hydroxylation in cases of adrenocortical tumors. Despite a still limited number of patients, the decreased B/DOC ratios may possibly indicate malig-nancy and could be helpful in distinguishing by biochemical means between benign and malignant adrenocortical tumors.