Data from the New South Wales Central Cancer Registry for the period 1972-1991 were examined to determine the risk of second primary cancers after an initial invasive cancer of the colon (ICD-9 153) or rectum (ICD-9 154). The expected numbers of cancers were obtained by assuming that subjects experienced the same cancer incidence as prevailed in the corresponding general population and by applying sex-, age-, and calendar-specific rates to the appropriate person-years at risk. The relative risk (RR) of a second primary cancer was taken to be the ratio of observed:expected numbers of second cancers. After colon cancer, there was an excess of cancers of the small intestine in both sexes (RRs of 4.5 and 4.4); prostate (RR = 1.4) and kidney (RR = 1.8) in men; and breast (RR = 1.3), body of uterus (RR = 1.9), ovary (RR = 2.8), and thyroid (RR = 2.7) in women. Lung cancer occurred less frequently in men than expected (RR = 0.7). After rectal cancer, men had increased risks of cancers of the colon (RR = 1.5) and prostate (RR = 1.3) and a reduced risk of pancreatic cancer (RR = 0.3). A reciprocal relationship of increased risk was seen between cancers of the proximal (but not the distal) colon and rectum. Shared luminal risk factors for proximal colon cancer and rectal cancer and three syndromes of hereditary predisposition to colon cancer seem to be the major contributors to second primary cancers in patients with an initial colon cancer. Sources of bias have been considered.