The association between the administration of oral antibiotics and cerebrospinal fluid (CSF) findings and sequelae was investigated in 281 children with Haemophilus influenzae type b meningitis from two prospective studies. Ninety-four (33%) children were pretreated; 59% of pretreated children received ampicillin or amoxicillin. Compared with untreated children, in pretreated children significant decreases were noted in the percentage of polymorphonuclear leukocytes in the CSF (P less than 0.03), CSF protein concentration (P less than 0.001) and percentage with a positive CSF Gram stain or culture (P less than 0.05). When adjusted for duration of illness prior to admission, only the CSF protein concentration remained different (P less than 0.01). Children who were pretreated were more likely (P less than 0.05) to have paresis at one or more follow-up visits and sensorineural hearing loss (P less than 0.05), but these differences were diminished when adjusted for duration of illness before admission. The duration of illness prior to admission was significantly (P less than 0.0001) longer for pretreated (median, 3.0 days) than for untreated children (median, 1.0 day). The incidence of deafness did not correlate with duration of illness before admission by multiple logistic regression analysis (P = 0.132), but deafness was significantly (P less than 0.02; relative risk, 5.9) more common when all children who were ill for more than 1 day prior to admission were compared to those children who were ill for 1 day or less.(ABSTRACT TRUNCATED AT 250 WORDS)