A series of 43 adrenocortical tumors was analyzed using nine histologic features. Mitotic activity, especially with atypical forms, and venous invasion correlated best with metastasizing or recurring tumors; however, no single criterion was useful alone. The combination of the following nine criteria was most useful in distinguishing malignant from benign tumors: nuclear grade III or IV; mitotic rate greater than 5/50 high-power fields; atypical mitoses; clear cells comprising 25% or less of the tumor; a diffuse architecture; microscopic necrosis; and invasion of venous, sinusoidal, and capsular structures. None of the 24 tumors with two or less of these criteria metastasized or recurred, while all but one of the 19 tumors with four or more of these criteria either recurred or metastasized.