A retrospective study of 72 infants under four weeks of age with meningitis evaluated over almost four years was performed to determine the predictive value of the CBC differential ratio (% lymphocytes + % monocytes/ % polymorphonuclear leukocytes + % band forms) profile in distinguishing those with bacterial from those with a nonbacterial etiology. Of 18 neonates with bacterial meningitis, all had a ratio lower than one; of 54 neonates with aseptic or viral meningitis, 46 (85%) had a ratio greater than one. The difference between mean ratio values of the two groups was highly significant (P < 0.001). A CBC differential ratio less than one was more sensitive (100%) in initially identifying neonates with bacterial meningitis than were other traditionally utilized parameters, eg, presence of fever (50%), ill appearance (50%), hypoglycorrhachia (61%), elevated CSF protein (55%), or CSF Gram-stained smear revealing pathogenic organisms (45%). Statistical analysis revealed that a CBC differential ratio less than 1.5 will accurately predict all cases of neonatal bacterial meningitis with a confidence of 99.95%. The CBC differential ratio is an accurate index for distinguishing neonates with bacterial meningitis from those with nonbacterial meningitis. Utilization of this parameter in conjunction with other traditional factors is efficacious in predicting outcome.