Introduction: Adenoma detection rate (ADR) is an established quality indicator for screening colonoscopy. The impact of fellow involvement in screening colonoscopy on ADR was conflicting in prior studies. In addition, we found no study assessing the effect of starting a new gastroenterology (GI) fellowship program on the quality of screening colonoscopies. In our practice, we started a GI fellowship program on July 1, 2012. The aim of this study was to assess the effect of starting a new GI fellowship and the participation of a fellow during colonoscopy on the ADR and other quality parameters of screening colonoscopy in a university setting. Methods: This is a retrospective, cross-sectional study on all screening colonoscopies done 20 months before and 20 months after starting the GI fellowship in our center (November 1, 2010 to February 28, 2014). Colonoscopy procedures and pathology records were reviewed for each patient. Continuous variables were described using mean and standard deviation, while categorical variables were described using frequencies and percentages. Fisher’s exact test was used to compare categorical variables while 2 sample t-tests were used to compare continuous variables. Results: A total of 2,226 screening colonoscopies performed by 10 endoscopists were obtained. The cecal intubation rate was 95.5%. The 2,127 complete colonoscopies were included in the analysis. The mean age of included patients was 58.8±6.6 years; 1,428 (67%) were females and 1,725 (81%) were Hispanic. In this time period, there were 1,072 (50.3%) procedures done before and 1,055 (49.7%) after starting the fellowship. Of the 2,127 colonoscopies, fellows were involved in 385 (18%), while 1,742 (82%) were performed by GI attendings alone. The overall polyp detection rate (PDR; 48.7 vs 36.4%; P<0.0001), ADR (34.1% vs 25.3%; P<0.0001), and advanced ADR (5.9% vs 3.7%; P=0.0203) were significantly higher in the period after starting the fellowship. In the period after starting the fellowship, the PDR and ADR for colonoscopies done by the attendings with the fellows were significantly higher versus colonoscopies performed by the same attendings alone (58.4% vs 44.5%; P=0.0005; and 42.0% vs 32.9%; P=0.0171, respectively). However, there was no significant change in advanced ADR (7.8% vs 6.7%; P=0.58). In addition, ADR was found to be higher with older age (P=0.0013), longer duration of procedure (P<0.0001), male gender (P<0.0001), presence of mass (P=0.0136), and AM colonoscopies (P=0.0138). Conclusion: In this retrospective study, we found that starting a GI fellowship program significantly increased the PDR, ADR, and advanced ADR. Additionally, fellow involvement in colonoscopy increased the ADR and PDR.