Group B streptococcal (GBS) disease is the most common cause of neonatal sepsis and meningitis in the United States. It is also an important cause of morbidity among pregnant women and adults with underlying medical conditions. Because most states have not designated GBS disease as a reportable condition, previous estimates of the incidence of GBS disease were based on studies from single hospitals or small geographic areas. This report summarizes the results of population-based active surveillance for invasive GBS disease in counties within four states that had an aggregate population of 10.1 million persons in 1990. A case of GBS disease was defined as isolation of group B streptococcus from a normally sterile anatomic site in a resident of one of the surveillance areas. Age- and race-adjusted projections to the U.S. population suggest that > 15,000 cases and > 1,300 deaths due to GBS disease occur each year. The projected age- and race-adjusted national incidence is 1.8/1,000 live births for neonatal GBS disease and 4.0/100,000 population per year for adult GBS disease. Intrapartum chemoprophylaxis for pregnant women at risk for delivering infants with GBS disease is the most effective strategy available for prevention of neonatal disease. Development of effective GBS vaccines may prevent GBS disease in both infants and adults. Ongoing surveillance for GBS disease is important for targeting preventive measures and determining their effectiveness.