OBJECTIVE Lumbar puncture can be an essential part of the septic work-up in premature infants who are at risk for sepsis and meningitis. Cerebrospinal fluid (CSF) values for cell counts, protein concentrations, and glucose concentrations in children and full-term infants are well established. CSF values in premature infants, however, have not been well studied. We sought to determine CSF values in very low birth weight premature infants at different ages (birth, postmenstrual age, and postnatal age). METHODS Medical records of all very low birth weight premature infants with suspected sepsis who were admitted to our neonatal intensive care unit between 1991 and 2005 were reviewed. Infants were excluded if they had evidence of intraventricular hemorrhage, sepsis/meningitis, or major congenital abnormalities or had a traumatic lumbar puncture. METHODS Neonatal intensive care unit. METHODS Patients were 455 infants who underwent lumbar puncture. METHODS None. RESULTS Medical records of 455 infants who underwent 648 lumbar punctures were reviewed. Of these, 243 infants met our inclusion criteria, and 88 patients underwent lumbar puncture only at birth. Patients' mean gestational age and birth weight were 28.8 +/- 2.6 wks and 1080 +/- 279 g, respectively. There were no correlations between gestational age and CSF white blood cell (WBC) count or between gestational age and CSF protein concentrations at birth. CSF WBC count remained unchanged at different postmenstrual ages. However, CSF protein concentration decreased with advancing postmenstrual age (Spearman's rho correlation coefficient, r = -.29; p < .01), and both CSF WBC count and CSF protein concentration decreased with advancing postnatal age (Spearman's rho correlation coefficient, r = -.319 and r = -.376, respectively; p < .01). A subgroup analysis revealed differences in CSF WBC count and CSF protein concentrations between infants who had a lumbar puncture at birth, at 2 wks, and at 3 wks of life at the same postmenstrual age. CONCLUSIONS In very low birth weight premature infants, CSF WBC count and CSF protein concentrations vary with advancing postnatal and postmenstrual ages.