Previous studies have demonstrated that during the first 24 hours of illness caused by aseptic meningitis, polymorphonuclear cells predominate in cerebrospinal fluid and decline afterward. To test the hypothesis that the absolute neutrophil count and percentage of polymorphonuclear cells in cerebrospinal fluid of bacterial meningitis patients are elevated and sustained after 24 hours compared with that of patients with aseptic meningitis. Seventy-two patients with aseptic meningitis and 13 with bacterial meningitis participated. All patients with aseptic meningitis and four patients with bacterial meningitis were followed-up prospectively. The patients were enrolled in four groups according to the interval between onset of symptoms and performance of the lumbar puncture (<12 hours, 12-24 hours, 24-36 hours, and >36 hours). In aseptic meningitis the mean absolute neutrophil count was 182, 164, 79, and 68 cells/mm3, respectively (P = 0.025). In bacterial meningitis the absolute neutrophil count was 28, 1,466, 5,853, and 235 cells/mm3, respectively. The mean percentage of polymorphonuclear cells in aseptic meningitis was 49%, 46%, 40%, and 26%, respectively (P = 0.038); in bacterial meningitis, 70%, 83%, 81%, and 58%, respectively. The findings suggest that the absolute neutrophil count and the polymorphonuclear cell percentage in cerebrospinal fluid have different kinetics in aseptic vs. bacterial meningitis.