Background: Second primary cancers are a major concern for young adult colorectal cancer (CRC) survivors. The impact of treatment with radiation on the risk for second solid and hematological malignancies, as well as the typical latency period before development of a second cancer, requires elucidation. Methods: The Surveillance Epidemiology and End Results (SEER) database was used to detect CRC cases in young adults (aged 20-40 years) diagnosed up to 12/31/2011. The Standardized Incidence Ratio (SIR) was calculated as the ratio of observed to expected cases of second primary malignancy based on incidence data in the general United States population. The latency exclusion period from the date of diagnosis was 5 years. We investigated the possible effect of radiation therapy on risk for a second cancer, and established the typical latency period after initial diagnosis (5-10 years and >10 years). Results: A total of 9,537 cases of CRC in young adults were available for analysis. Radiation was administered in 1,823 (19.1%) patients. Second primary cancers occurred in 798 cases, 754 (94.5%) solid tumors and 44 (5.5%) hematological malignancies respectively. In the latency period 5-10 years after initial diagnosis, “All Solid Tumors” (SIR: 1.85, p<0.05) and female genital tumors (SIR: 3.52, p<0.05) were increased. In the latency period >10 years after initial diagnosis, an increased risk of developing pancreatic cancer (SIR: 1.90, p<0.05) and female genital tumors (SIR: 1.77, p<0.05) was observed. In patients who received radiation the only significantly increased risk was for “All Solid Tumors” (SIR: 1.42, p<0.05). No significant increase in second hematological malignancies was observed in the entire cohort, including cases that received radiation. Conclusions: An increased risk of second primary female genital tumors was seen in both the 5-10 years and >10 years latency periods. Pancreatic cancer risk was increased specifically in the >10 years latency period. Radiation treatment did not pose an increased risk for any specific second primary cancers. Interestingly, young adult CRC survivors did not have an increased risk for hematological malignancies compared to the general population.