Histological slides of the adrenals of 54 patients who underwent uni- or bilateral adrenalectomy for Cushing's syndrome at the Zürich University Hospital Department of urology between 1960 and 1977 were reexamined. In 29 patients (53.7%) Cushing's disease was due to diffuse bilateral adrenocortical hyperplasia, in 4 (7.4%) to micronodular cortical hyperplasia and in 3 (5.6%) to macronodular cortical hyperplasia and in 3 (5.6%) to macronodular hyperplasia. Two patients (3.7%) presented with primary bilateral nodular cortical dysplasia (so-called microadenomatosis). In 12 cases of Cushing's syndrome (22.2%) the basic lesion was a cortical adenoma and in 4 cases (7.4%) a cortical carcinoma. The histologic diagnoses corresponded well with biochemical findings, clinical diagnoses and further development of the disease. Histopathologic diagnosis of diffuse and micronodular hyperplasia presents no difficulties. Differential diagnosis of macronodular hyperplasia, cortical dysplasia, adenoma, and carcinoma may, however, be somewhat more problematic.