Patients who had a colorectal cancer have a 1.5‐ to 2‐fold excess risk of a second colorectal cancer as compared to the general population, the excess being higher at younger age at diagnosis. To further investigate the risk and the age‐relation of the incidence of second primary colorectal cancer, we considered 9,389 first colon and rectal cancers registered in the Vaud Cancer Registry, Switzerland, between 1974 and 2008, and followed‐up to the end of 2008 for a total of 44,113 person‐years. There were 136 second colorectal cancers versus 90.5 expected, corresponding to a standardized incidence ratio (SIR) of 1.5 (95% confidence interval, CI, 1.3–1.8). The SIRs were not heterogeneous between men and women, and in strata of calendar year at diagnosis, duration of follow‐up, and subsite. However, the SIR was 7.5 (95% CI 4.2–12.4) for subjects diagnosed below age 50 and declined thereafter to reach 1.0 (95% CI 0.6–1.6) at age 80 or over. Consequently, the incidence of second primary colorectal cancer was stable, and exceedingly high, around 300–400/100,000 between age 30–39 and 70 or over. This age pattern is consistent with the existence of a single mutational event in a population of highly susceptible individuals.