BACKGROUND: Local excision may be curative for benign and malignant rectal neoplasms. Because many early rectal cancers are discovered incidentally after local excision of clinically benign lesions, it is unclear whether preoperative imaging with transrectal ultrasound or MRI affects management. OBJECTIVE: The purpose of this study was to determine the diagnostic characteristics and effect of preoperative imaging on the incidence of malignancy in benign rectal lesions undergoing local excision. DESIGN: Prospective data from 2 institutions were included. Coarsened exact matching created a balanced cohort comparing imaging and no-imaging groups. SETTING: The study was conducted at high-volume specialist referral hospitals. PATIENTS: Adult patients undergoing local excision via transanal endoscopic surgery between 1997 and 2016 for clinically benign rectal neoplasms were included. INTERVENTION: The study intervention included preoperative imaging with transrectal ultrasound and/or MRI. MAIN OUTCOME MEASURES: We measured the incidence of malignancy and diagnostic accuracy of preoperative imaging. RESULTS: A total of 620 patients were included (272 with preoperative imaging and 348 without). There were 250 patients undergoing transrectal ultrasound, and 24 patients undergoing MRI (2 patients underwent both). Transrectal ultrasound and MRI correctly identifed malignant polyps in 50% (11/22) and 44% (8/18). Overall agreement for benign versus malignant polyps between preoperative imaging and final pathology was κ = 0.30 (95% CI, 0.18-0.41) for transrectal ultrasound and 0.29 (95% CI, 0.01-0.57) for MRI. In both the overall and unmatched cohorts, the incidence of malignancy, margin involvement, and proportion of patients requiring salvage surgery was similar. LIMITATIONS: Data were obtained from 2 institutions with different equipment over a long time period. CONCLUSIONS: Preoperative imaging did not accurately identify malignancy in clinically benign rectal lesions and did not affect the incidence of malignancy, margin involvement, or proportion of patients requiring salvage surgery. Therefore, preoperative imaging may not be necessary for clinically benign lesions undergoing local excision.