Cerebrospinal fluid samples sterile for virus and bacteria from 371 patients who had no evidence of CNS pathology were examined. The patients were divided into five age categories. The white blood cell polymorphonuclear, and mononuclear cell counts were evaluated. The frequency distributions for these cell counts were markedly skewed, so values were calculated as percentiles for each age group. WBC values were highest for CSF obtained from patients in younger age categories, but 25% of patients from all age categories had two or three WBCs in their CSF. Patients from all but the 3- to 6-month category had three polymorphonuclear cells in their CSF 5% of the time. There was no significant difference in CSF WBC count between patients who had seizures and those who did not. Knowledge of normal values in different age groups is needed for the interpretation of CSF findings, but it should be used in conjunction with other clinical and laboratory findings.