PURPOSE:The purpose of this study was to assess risk of developing multiple primaries after a diagnosis of colon cancer and to determine the impact that having a family history of cancer has on cancer risk.METHODS:Data from the Utah Cancer Registry and the Utah Population Database were used. A cohort of 2,236 first primary colon cancers were observed for the subsequent development of additional primary cancers.RESULTS:We observed a greater than expected incidence of colon, rectal, and pancreatic cancers among the cohort. The standardized incidence ratios were 2.77 (95 percent confidence interval (CI), 2.07‐3.70), 2.26 (95 percent CI, 1.34‐3.81), and 2.38 (95 percent CI, 1.32‐4.30), respectively. Having a family history of colon or rectal cancer did not greatly influence risk of having a multiple primary. However, there was a trend toward increased risk of pancreatic cancer (hazard rate ratios, 1.99; 95 percent CI, 0.67‐5.90) and bladder cancer (hazardrate ratios, 2.35; 95 percent CI, 0.77‐7.18) among patients with a family history of rectal cancer. We also observed that risk of uterine cancer in the cohort was positively associated with family history of uterine cancer, risk of breast cancer was positively associated with family history of breast cancer, and risk of prostate cancer was positively associated with family history of prostate cancer.CONCLUSIONS:People with colon cancer are at a greater risk of developing colon, rectal, and possibly pancreatic cancer. Although a family history of colon or rectal cancer did not have a large impact on developing other cancers, a family history of other primary cancers did influence risk of other cancers.