Uterine rupture (UR), a potentially life-threatening condition for both mother and infant, occurs in <0.1% of all pregnant women and <1% of women attempting vaginal birth after cesarean section (VBAC) (1-4). During 1990-1997, the proportion of vaginal deliveries among women who had previous cesarean sections (CS) in Massachusetts increased 50%, from 22.3% to 33.5% (5). Concern about a corresponding increase in UR prompted the Massachusetts Department of Public Health and CDC to initiate a state-wide investigation that included an assessment of the validity and reliability of International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) (6), codes in hospital discharge data to identify UR cases. This report summarizes the results of the investigation, which indicate that ICD-9-CM codes related to UR, designed before increased concern about UR, lack adequate specificity for UR surveillance and have not been applied consistently over time.