In 2002, CDC, the American College of Obstetricians and Gynecologists (ACOG), and the American Academy of Pediatrics (AAP) issued revised guidelines for prevention of perinatal invasive group B streptococcal (GBS) disease. These guidelines recommend universal screening of pregnant women for rectovaginal GBS colonization at 35-37 weeks' gestation and administering intrapartum antimicrobial prophylaxis to carriers. To assess the impact of the guidelines on multistate trends in neonatal GBS disease incidence, CDC analyzed data from the Active Bacterial Core surveillance (ABCs) system from 1996-2004. This report summarizes the results of that analysis, which determined that incidence of GBS disease in infants aged 0-6 days (i.e., early-onset disease) in 2004 had decreased by 31% from 2000-2001, the period immediately before universal screening was implemented. Incidence of GBS disease in infants aged 7-89 days (i.e., late-onset disease) remained unchanged during the 9-year period reviewed. Continued monitoring is needed to assess the impact of the 2002 guidelines on early-onset disease and the long-term effect of widespread intrapartum use of antimicrobial agents on neonatal GBS disease.