Background: National guidelines (NCCN 2016) recommend profiling for microsatellite instability (MSI) and mismatch repair (MMR) for all patients (pts) with colon cancer. Evaluations may identify pts with familial syndromes requiring surveillance. Profiling may assist in clinical decision making: MSI-High have a significantly better prognosis, stage II pts with defective MMR may not benefit from fluorouracil treatment, pts with defective MMR may harbor BRAF mutations, and advanced stage MSI-High pts may be candidates for immunotherapy. Adherence to testing guidelines is unknown. Methods: The Cota database aggregates detailed diagnostic, clinical, treatment, and outcome information from the electronic health records. Limitations of the database include failure to capture diagnostic tests not scanned into the EHR. Results: 1978 pts with colon cancer diagnosed Jan, 1, 2013 through Dec 31, 2017 from 22 institutions (treated by 173 oncologists) were retrospectively reviewed. 975 (49%) had documentation of immunohistochemical testing for an MMR protein and 343 (17%) had polymerase chain reaction based testing for MSI. (MMR testing by stage: I: 49%; II: 54%; III: 51%; IV: 49%). Testing rates before 2016 for MMR were 47%, which increased to 54% following guideline publication (p = 0.001). Academic centers tested at 67% (400/593) vs 41% (575/1385) in community cancer centers (p < 0.0001). The 18 oncologists evaluating ≥20 pts tested at 52% (422/805) vs 46% (424/925) for physicians with smaller practices. (p = 0.007). Defective MMR were noted in MSH2 5% (45/930), MSH6 5% (46/913), MLH1 16% (137/833), and PMS2 16% (132/821). Testing for BRAF was 14% in the series. Among pts with defective PMS2, 30% were tested for BRAF mutations and 44% were positive (vs 18% testing rate if PMS2 intact with 14% positive). MLH1 pts were BRAF tested 27% with 39% positive. Among the 387 Stage IV pts, 60% were tested for KRAS (37% positive) and 23% for NRAS (5% positive). Conclusions: Testing for MSI/MMR is below guideline recommendations, but is increasing and is higher at academic centers and by physicians with larger colon cancer populations. Given the implications of MSI/MMR educational efforts are needed.