The histologic appearances of ductal invasion were studied in 139 cases of prostatic adenocarcinoma diagnosed in the Department of Pathology, Howard University Hospital, during the period January 1980 through October 1983. Intraductal spread was found in almost half (48%) of the prostatic glands examined. Ductal spread was associated with the local extent (P less than 0.001) rather than with the grade of the tumor (P less than 0.01). Three distinct patterns of ductal penetration were recognized. The duct wall was completely destroyed in microinvasion. In foci of ductal permeation the integrity of the basement membrane was generally preserved, and the duct wall was infiltrated mainly by solitary tumor cells. When the tumor spread was by extension in continuity within the duct wall, the neoplastic cells appeared to grow between the pre-existing epithelial layers. It was concluded that prostatic carcinoma cells have the ability to penetrate the wall of benign ducts and progressively replace the normal epithelial elements. In this process the general framework of the affected duct appears to be preserved.