Three-hundred and forty-nine cases of acute bacterial meningitis treated during a 25-year period (1949 through 1973) were reviewed to determine the prognostic significance of initial historical, physical and laboratory findings. A poor prognosis was associated with age greater than or equal to 40 years (p less than 0.01), presence of predisposing illness (p less than 0.01), associated illness (p less than 0.01), absence of nuchal ridity (p less than 0.05), and derangement of cerebral function (p less than 0.01). The effects of predisposing illness and moderate cerebral dysfunction were dependent upon age. In contrast, the effects of associated illness, mild or severe cerebral dysfunction, and absent nuchal rigidity were independent of age. Laboratory studies associated with a poor prognosis included an elevated cerebrospinal fluid (CSF) protein (p less than 0.05), a positive CSF smear (p less than 0.05), or culture (p less than 0.01), or bacteremia (p less than 0.01). No prognostic significance could be attributed to race (p greater than 0.05), sex (p greater than 0.05), prior antibiotic therapy (p greater than 0.05), duration of illness before institution of adequate therapy (p greater than 0.05), CSF leukocyte count (p greater than 0.05), frequency of polymorphonuclear leukocytes in CSF (p greater than 0.05), CSF sugar less than or equal to 40 mg/100 ml (p greater than 0.05), or a CSF sugar-simultaneous blood sugar ratio less than or equal to 0.40 (p greater than 0.05).