OBJECTIVE: To determine the error rate for discharge coding of anal sphincter laceration at vaginal delivery in a cohort of primiparous women. METHODS: As part of the Childbirth and Pelvic Symptoms study performed by the National Institutes of Health Pelvic Floor Disorders Network, we assessed the relationship between perineal lacerations and corresponding discharge codes in three groups of primiparous women: 393 women with anal sphincter laceration after vaginal delivery, 383 without anal sphincter laceration after vaginal delivery, and 107 after cesarean delivery before labor. Discharge codes for perineal lacerations were compared with data abstracted directly from the medical record shortly after delivery. Patterns of coding and coding error rates were described. RESULTS: The coding error rate varied by delivery group. Of 393 women with clinically recognized and repaired anal sphincter lacerations by medical record documentation, 92 (23.4%) were coded incorrectly (four as first- or second-degree perineal laceration and 88 with no code for perineal diagnosis or procedure). One (0.3%) of the 383 women who delivered vaginally without clinically reported anal sphincter laceration was coded with a sphincter tear. No women in the cesarean delivery group had a perineal laceration diagnostic code. Coding errors were not related to the number of deliveries at each clinical site. CONCLUSION: Discharge coding errors are common after delivery-associated anal sphincter laceration, with omitted codes representing the largest source of errors. Before diagnostic coding can be used as a quality measure of obstetric care, the clinical events of interest must be appropriately defined and accurately coded. © 2007 The American College of Obstetricians and Gynecologists.