Background: Patients with hereditary non polyposis colorectal cancer (HNPCC) diagnosed with CRC have an elevated risk of a second primary CRC (SPCRC) and of rapid primary cancer development. This informs both initial surgical approach and endoscopic surveillance intervals. A feature of HNPCC is dMMR, also found in 15% of sporadic CRC, where the risk of SPCRC has yet to be defined. Methods: We examined a multi-site comprehensive CRC database (Melbourne, Australia), where prospectively collected data includes family history (including HNPCC), histology, surgery performed and incidence of second primary cancer. Sporadic dMMR included any case with a BRAF V600E mutation, confirmed hypermethylation, negative germline testing, or age over 60 years. We explored the incidence and timing of SPCRC in patients with early stage sporadic dMMR (or MSI-H) versus pMMR cancers. Results: From February 2004 to December 2019, 7442 patients diagnosed with stage I-III CRC were recorded. MMR status was known in 4079 (54.8%), including 714 with dMMR colorectal cancer (17.4%) of which 575 (14.6%) were deemed sporadic dMMR. Sporadic dMMR patients were older (mean 76.2 years vs 65.9 years, p = < 0.001), more likely to be female (65.2% vs 42%, p = < 0.001) and have a right sided primary (80.9% vs 32.8%, p = < 0.001), compared to patients with pMMR CRC. A SPCRC was diagnosed in 11/575 patients (1.91%) with sporadic dMMR CRC versus 27/3365 (0.83%) patients with pMMR CRC (HR = 2.57, 95% CI 1.28 - 5.17, p = 0.008). Median time to SPCRC was 1.13 years vs 2.38 years (p = 0.49). The SPCRC diagnosed in sporadic dMMR CRC vs pMMR CRC were more likely to be dMMR ((72.7%) vs (25.9%), p = 0.03), but a similar number were stage I or II ((81.8%) vs (81.5%)) and a similar number were surveillance detected ((72.7%) vs (77.8%). Conclusions: Patients with sporadic dMMR appear to have a significantly elevated risk of SPCRC compared to the pMMR population and were diagnosed at a shorter interval. The SPCRC is also more likely to also be dMMR. Increased colonoscopic surveillance of patients presenting with an initial sporadic dMMR cancer should be considered where clinically appropriate.