A prospective, laboratory-based surveillance project obtained accurate data on meningitis in a population of 34 million people during 1986. Haemophilus inftuenzae was the most common cause of bacterial meningitis (45%), followed by Streptococcus pneumoniae (18%), and Neisseria meningitidis (14%). Rates of H. inftuenzae meningitis varied significantlybyregion, from 1.91100,000 in New Jersey to 4.0/100,000 in Washington state. The overall case fatality rates for meningitis were lower than those reported in several studies from the early 1970s, suggesting that improvements in early detection and antibiotic treatment may have occurred since that time. Concurrent surveillance was also performed for all invasive disease due to the five most common causes of bacterial meningitis. Serotypes of group B streptococcus other than type III caused more than halfof neonatal group B streptococcal disease and mortality, suggesting that an optimal vaccine preparation must be multivalent. Of the organisms evaluated, group B streptococcus was the second most common cause of invasive disease in persons >5 years old.