Histologic study of about 700 biopsy specimens from patients with prostate cancers collected by the Radiation Therapy Oncology Group (RTOG) reemphasized the difficulty in accurately predicting biologic behavior of the carcinoma in individual cases. Some well-differentiated carcinomas metastasized to regional lymph nodes. Histologic proof of lymph node metastases may be difficult to detect. Some duct carcinomas do not readily fit a Gleason pattern. A variety of histologic grades may be seen either in a single sample or after the passage of time in the same case. Some prostatic epithelial proliferations may closely simulate carcinomas by routine light microscopy, but are biologically benign. Some of these prostate lesions have histologic similarities to breast lesions.