366 Background: Lynch syndrome (LS) is caused by germline mutations in DNA mismatch repair (MMR) genes and increases the risk of colon and several other cancers. Prostate cancer is not currently considered part of the LS-tumor spectrum. The objective of this retrospective study was to assess whether the incidence of prostate cancer is increased above that of the general population in MMR mutation carriers. Methods: Patients who were identified as MMR mutation carriers and included in previous LS studies or evaluated at the genetics clinic at The Ohio State University were contacted and their cancer history documented. Follow-up time was defined as the time between their first index cancer or their entry onto the LS studies until the last date of follow-up or date of death, whichever came first. The prostate cancer incidence was adjusted for age and race and compared to that of the SEER registry of 1999-2009. A standardized rate ratio (SRR) was obtained by comparing the observed rate to the expected rate. Results: 186 males (median age 49 years, range 17-83) were identified as MMR mutation carriers either by testing (95%) or as obligate carriers (5%) in 94 families. Their mean follow-up time was 5.1 years (SD+/-6.0 years) with a total of 938 person-years (p-ys). Caucasians comprised 95.1% of the cohort, African-Americans 3.8% and Asians 1.1%. Of the patients who were tested for mutations, MSH2 was most commonly mutated (49.2%), followed by MLH1 (23.7%), PMS2 (14.7%) and MSH6 (12.4%). Ten patients (5.4%), all Caucasian, were diagnosed with prostate cancer at a median age of 59 years (range 52-82). 7 patients were identified as MSH2 carriers, 2 as MSH6 carriers and 1 as a PMS2 carrier. The observed incidence rate was 102.8 cases per 100,000 p-ys versus the expected number of 158.2 cases per 100,000 p-ys, the standardized rate ratio was 0.65 (95% CI 0.53-0.79). Conclusions: Prostate cancer incidence was not increased in this relatively large cohort of LS patients.