Background: Recent studies have shown the adenoma detection rate (ADR) to decrease from endoscopist fatigue. Objective: Our primary objective was to evaluate the afternoon ADR in half-day versus full-day blocks. Secondary objectives were to determine whether time or complexity of prior procedures (modified queue position) influence ADR. Design: Retrospective chart review on consecutive colonoscopies. Setting: Tertiary-care teaching hospital. Patients: This study involved all patients in our database who were over age 45 and who underwent screening and surveillance colonoscopies. Main Outcome Measurement: ADR. Results: A total of 3085 patients were included in the study, with an overall 31% ADR. Of these procedures, 2148 (70%) were done in the morning, and 937 (30%) were done in the afternoon (512 full-day block, 425 half-day block). By multivariate analysis, there was no difference in ADR between full-day blocks compared with afternoon-only blocks (35% vs 32%; odds ratio [OR] 1.1; 95% confidence interval [CI], 0.8-1.5; P =.2). For all afternoon colonoscopies, no decrease in ADR was noted with increasing queue position (P =.9) or time (P =.3). In addition, no difference was found comparing ADR between all afternoon colonoscopies versus morning colonoscopies (33% vs 30%; OR 1.1; CI, 1.0-1.3; P =.1). No difference was found for advanced adenomas and number of adenomas between either afternoon-only blocks versus afternoon colonoscopy in full-day blocks or morning versus all afternoon cases. Limitations: Retrospective study; not all withdrawal times were recorded; trainees performed some of the procedures. Conclusion: Our data show that colonoscopy can be performed in full-day blocks and 30-minute slots without compromising ADR.