Lumbar puncture frequency and cerebrospinal fluid analysis in the neonate. A prospective study was performed to assess the frequency and diagnostic utility of lumbar punctures in neonates both during their first week of life and thereafter. During the two 6-month periods from January 1, 1985 to June 30, 1985, and February 1, 1986 to July 31, 1986, 712 neonates underwent 728 lumbar punctures during their first week of life primarily as part of the evaluation for suspected infection, either congenital or postnatal. There were eight patients with positive spinal fluid cultures in the first week of life, but only one patient simultaneously had a positive blood culture and a clinical course consistent with meningitis. In contrast, a considerably higher yield, approximating five times that of the first week of life, was obtained in patients undergoing a lumbar puncture after the first week of life. 