To assess the compensatory functional and anatomic changes in the remaining adrenal cortex after unilateral adrenalectomy or in the unaffected adrenal in patients with unilateral adrenal destruction by neoplasm, 17 patients with a single, functioning adrenal gland and normal indices of adrenocortical function, nine after adrenalectomy and eight with a unilateral, destructive adrenal lesion were studied with 131I-6 beta-iodomethyl-19-norcholesterol (NP-59) scintigraphy and computed tomography. Adrenal masses with a mean (+/- s.d.) diameter of 2.8 +/- 1.0 cm; (range 1-4 cm; 95% confidence interval (Cl), 2.5-3.1 cm) were identified by computed tomography in seven of nine patients in the remaining adrenal cortex at variable times (6.1 +/- 5.9 y; range 0.5-19 y) after unilateral adrenalectomy. Mean (+/- s.e.m.) NP-59 uptake was elevated (p less than 0.01) in both adrenalectomy and adrenal destruction groups, mean uptake (+/- s.e.m.) was 0.32% +/- 0.04% administered dose (95% Cl, 0.24%-0.4% administered dose) as compared to normal (0.16% +/- 0.05% administered dose, 95% Cl, 0.06%-0.26% administered dose). The remaining adrenal cortex may be anatomically abnormal after unilateral adrenalectomy and demonstrate compensatory, increased NP-59 uptake in the presence of overall, normal adrenocortical function.