Increased frequencies of adrenal tumours and testicular adrenal rest tumours (TART) have been reported in patients with 21-hydroxylase deficiency (21OHD). Patients, methods and design From a cross-sectional population-based study of 101 adult Norwegian patients with 21OHD, sixty-two participated in this study (23 men, 39 women; age range 18-75); thirty-two were salt wasting (SW) and 30 simple virilizing (SV); they were assessed with adrenal computed tomography (CT), testicular ultrasound and hormone measurement in the morning after overnight medication fast. Nine adrenal tumours were detected in seven (11%) patients (bilateral in 2); four were myelolipomas and one a phaeochromocytoma. Seventeen (27%) had normal adrenal size, whereas 36 (58%) had persisting hyperplasia, and seven (11%) adrenal hypoplasia. Abnormal adrenals were more common in SW than in SV. TART occurred exclusively in SW and was present in seven (57%) of these men. Testicular volumes were small compared with normative data. Morning ACTH and 17-hydroxyprogesterone levels correlated positively with adrenal dimensions and frequency of TART. In this unselected population of patients with classical 21OHD, we found high frequencies of adrenal tumours, particularly myelolipomas, and of hyperplasia and hypoplasia, and TART in SW. It is important that physicians are aware that benign adrenal and testicular tumours occur frequently in 21OHD. Furthermore, these findings may reflect inappropriate glucocorticoid therapy, making a case for the advancement of novel physiological treatment modalities. © 2011 Blackwell Publishing Ltd.