DNA ploidy pattern in rectal carcinoid tumors. The nuclear DNA pattern of 22 rectal carcinoids was determined by cytophotometry of paraffin embedded tissues. The results were compared with clinical as well as histopathologic features of the tumor. Three of the carcinoids with synchronous or metachronous metastasis had aneuploid DNA pattern, whereas 19 tumors with no metastasis showed diploid DNA pattern. No other single clinical or pathologic feature of the tumor could predict more accurately the malignant potential and the subsequent course of the rectal carcinoid. It is concluded that DNA aneuploidy in rectal carcinoid tumors is not so rare as indicated by earlier studies and that it is a factor of significant prognostic value. 