The occurrence of a second primary malignant disease was analyzed in 38,166 patients with cancer of the colon and 23,603 patients with rectal cancer reported to the Swedish Cancer Registry between 1960 and 1981. The overall relative risk (RR) of developing a second primary malignant disease was significantly (P less than 0.05) increased both after cancer of the colon (women, RR = 1.4; men, RR = 1.3) and rectum (women, RR = 1.4; men, RR = 1.3). Besides confirming an increased risk of metachronous colorectal cancer this study suggests that cancer of the small intestine, breast, endometrium, and possibly of the ovary and prostate may have etiologic factors in common with cancer of the large bowel, notably those located in the colon.