Cerebrospinal fluid (CSF) sterilization after greater than 24 h of intravenous antibiotic therapy (delayed CSF sterilization) was noted in two infants treated with ceftizoxime and ceftazidime for bacterial meningitis. A case-control study was conducted of children between 6 w and 6 y of age treated between 1975 and 1985 at one institution for bacterial meningitis to determine risk factors for delayed CSF sterilization. Hemophilus influenzae type b was isolated from all children (n = 5) with delayed CSF sterilization, compared with only 78% of all children in the study (n = 83). In children with H. influenzae type b disease, children less than 6 mo of age were at higher risk than older children for delayed CSF sterilization (odds ratio = 7.5, 95% confidence limits = 1.4, 40.0). Factors not associated with delayed CSF sterilization included time of follow-up lumbar puncture, CSF total or differential white blood cell count, and CSF protein and glucose concentrations. Despite the in vitro antimicrobial susceptibility of H. influenzae type b to ceftizoxime and ceftazidime, delayed CSF sterilization may occur in infants receiving these antibiotics for bacterial meningitis.