Background: uMMR of CRC/EC for Lynch syndrome (LS) is recommended to identify the 2-3% of incident CRC/EC with germline MMR mutations in MLH1, MSH2, MSH6, and PMS2. Many US centers use a positive immunohistochemical (IHC+) tumor test to prompt further evaluation such as genetic counseling (GC). Efficacy of uMMR is established, but real-world performance is not. W/recent Blue Ribbon Panel recommendations for broad implementation, we reviewed uMMR effectiveness in an academic cancer center. Methods: Outcomes of all CRC/EC undergoing uMMR from 9/2011 (start of uMMR at Fox Chase Cancer Center) to 12/2017 were reviewed. uMMR results are reported in the surgical pathology report and by email to providers w/a PDF test report meant for pts. Results [IHC, MSI, BRAF, MLH1 promoter methylation, gene testing] and clinical data were extracted from a clinical genetics database and the EMR. Associations were tested by chi-square and T-test (2-sided). Results: From 1156 screened tumors, 278 (24.0%) had +IHC: 112 (40.3%) CRC vs 166 (59.7%) EC. Mean age was 64.7 yrs, and 117/278 (42.4%) reported family history (FamHx) of a LS cancer. After BRAF V600E testing, 91.0% (253/278) warranted GC. Only 98/253 (38.7%) pts warranting GC have to date completed GC (mean follow-up 3.0 yrs)—most common pt-reported reason for non-compliance w/GC was low interest (46.7%). Pts who completed GC were younger (47.9% vs 29.6% were < 60 yrs, p = 0.004), and had stronger FamHx (55.1% vs 34.8%, p = 0.006), while differences in GC uptake by race (40.6% White vs 28.2% non-White, p = 0.14) were not significant. Of the 67 pts who sought counseling and warranted a germline test, 91.8% completed testing. LS was diagnosed in 20.9% (14/67) or 1.1% of the uMMR population—nearly 1 in 4 tested (24.6%, 17/67) were clinically suspicious for LS but test results were inconclusive. Conclusions: uMMR effectiveness is limited by low uptake of GC as well as high rates of non-diagnostic test results. Among uMMR IHC+ pts, low interest is the foremost patient barrier to GC uptake, and is strongly influenced by personal (age) and familial risk factors. Molecular testing alone (IHC+ screen) may be insufficient to achieve desired uMMR outcomes.