Objective: To evaluate whether final ostium size is determined by the osteotomy created during dacryocystorhinostomy (DCR). Design: Prospective nonrandomized study. Intraoperative measurements of bony osteotomy were taken during external DCR. Endonasal endoscopy with functional endoscopic dye testing and internal ostium photography were performed 3 months after surgery. Results: Fifty patients (mean age, 63 years) underwent 55 DCRs. Postoperative nasal endoscopy with functional endoscopic dye testing was performed in 27 cases (49%), and measurements of intranasal ostia were feasible in 24 of them (86%). The mean follow-up time was 7 months (range, 3-12 months). Surgical success was achieved in 25 of 27 patients (93%) who underwent postoperative nasal endoscopy. There was no difference in either the intraoperative osteotomy size or the postoperative ostium size between failed and successful cases. The mean (SD) intraoperative osteotomy size was 256.3 (89.0) mm 2, and the mean (SD) postoperative ostium size was 9.6 (6.7) mm 2. The intraoperative osteotomy size correlated positively with the postoperative intranasal ostium size (r=0.45; P=.03, Pearson bivariate correlation). Conclusions: Larger osteotomies created during external DCR are correlated with larger postoperative ostia as measured by endonasal endoscopy and image analysis software. There is a trend toward greater success with larger osteotomies; however, failed cases in this series were not associated with smaller-sized intraoperative osteotomies.