Eighty-five cerebrospinal fluid (CSF) specimens from the same number of pediatric patients with meningitis were examined to determine the bacterial count and the relationship of this count to the microscopy results, the ages of the patients and the bacterial species isolated. Bacterial counts ranged from 2 x 10 to 4 x 10(9) CFU/ml CSF. Twenty-five percent of the 85 CSF specimens positive for Haemophilus influenzae type b, Neisseria meningitidis, Streptococcus pneumoniae, Escherichia coli K1 and group B streptococci had counts of 10(7) CFU/ml or higher. Children between 1 and 6 months of age had significantly higher counts (p less than 0.05) than the other age groups. The three patients who had positive CSF cultures 24 h after the start of therapy all had initial bacterial counts of 10(7) CFU/ml or higher. The detection limit for Gram stain/microscopy was 10(5) CFU/ml. No correlation was found between bacterial count and the number of polymorphonuclear leukocytes.