BACKGROUND: Most clinical overviews of acute bacterial meningitis have either focused on children or all age groups combined, although the disease poses serious problems in the adult population. OBJECTIVE: To study the clinical and microbiological features of adult bacterial meningitis in Iceland, as a representative of the average European or North American community. PATIENTS AND METHODS: Data on a total of 132 cases in 127 patients (age, > or = 16 years) who were diagnosed as having acute bacterial meningitis in Iceland during the years 1975 to 1994 were collected from patient and laboratory records. Complete hospital records were found for 119 of the 132 cases identified. RESULTS: The annual incidence was 1.7/100,000 to 7.2/ 100,000 inhabitants (mean, 3.8/100,000). The most common causative organisms were Neisseria meningitidis (56%), Streptococcus pneumoniae (20%), Listeria monocytogenes (6%), and Haemophilus influenzae (5%). Neisseria meningitidis caused 93% of the infections in the 16- to 20-year-old age group, but it caused only 25% of the infections in patients aged 45 years or older. Listeria monocytogenes caused 14% of these cases. Cases of nosocomial and recurrent meningitis were rare. A significant underlying illness or condition was present in 39% of the patients. The mean mortality was 19.7%, and it did not change during the study period. CONCLUSIONS: In a study that involved all adult patients with bacterial meningitis in a single country for 2 decades, meningococci and pneumococci were the most frequent causative agents. However, meningococci were responsible for only one fourth of the cases among adult patients aged 45 years or older, most of these cases were caused by pneumococci and Listeria. Despite modern medical developments, approximately 20% of adult patients with bacterial meningitis died.