The investigation was concerned with histological and ultrastructural features of adrenocortical tumors of the adrenals, which have a differential-diagnostic and clinico-prognostic relevance. Histological and electron-microscopical examination of 60 tumors (adrenocortical adenoma--12; adrenocortical cancer--48) was carried out and the findings were compared with clinical data. No significant correlation between histological pattern of adrenocortical tumors and survival was established. Ultrastructural evidence showed electron-microscopical examination to be a reliable procedure for making prognosis of adrenocortical cancers. There was a direct correlation between the level of differentiated cells and degree of their differentiation, on the one hand, and the quality of prognosis and survival, on the other. The significant predominance of differentiated cells was characteristic of adrenocortical adenomas.