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Colorectal cancer (CRC) patients have an increased risk of developing other malignancies. Understanding the characteristics of the second primary cancer is important to establish an effective surveillance program.This study investigated 301 CRC patients to assess the risk factors for postoperative primary cancers arising from organs distinct from the colorectal area (extracolorectal cancers). The observed/expected ratio (O/E ratio) was calculated using the Osaka Cancer Registry, to determine the rate of increase in extracolorectal cancers.The frequency of postoperative extracolorectal cancers was 12.6%. A logistic regression analysis showed only age to be an independent risk factor for postoperative extracolorectal cancer development. The O/E ratio of overall postoperative extracolorectal cancer was significantly higher than one (O/E ratio 2.6, p < 0.01). In each organ, the frequency of lung and gastric cancers were significantly higher than one, with O/E ratios of 3.2 and 2.7 (p < 0.01 and p < 0.05, respectively).The frequency of postoperative extracolorectal cancers in CRC patients was significantly higher than that in the normal population, especially for lung and gastric cancers. Clinicians should carefully follow patients for a possible recurrence of CRC and educate CRC patients with regard to the high risk of a second primary cancer.