OBJECTIVE This study examines the beta-glucuronidase activity in the cerebrospinal fluid (CSF) of patients with urinary tract infection (UTI) and sterile CSF pleocytosis and the feasibility of using these measurements for diagnostic purposes. METHODS beta-Glucuronidase activity was measured in the CSF from 22 in each group neonates and infants with UTI and sterile CSF pleocytosis, bacterial meningitis, aseptic meningitis of apparently viral etiology and controls without CSF pleocytosis. RESULTS The median (range) beta-glucuronidase activity in UTI with sterile CSF pleocytosis was 44.1 (33.2-57.1), whereas in the controls without CSF pleocytosis it was 19.1 (7.0-22.7), in aseptic meningitis of apparently viral etiology it was 26.5 (21.0-30.0) and in bacterial meningitis it was 168 (70.0-1152). The difference between the enzyme activity in the CSF of the patients with UTI and those in the other groups of neonates and infants is significant (p < 0.0001), with no overlapping between UTI and the other groups of children studied. Both the sensitivity and specificity of the activity was 100%. Conversely, there was a broad overlapping of the classic CSF laboratory parameters among the groups of subjects studied. CONCLUSIONS beta-Glucuronidase activity in cell-free CSF discerns, with much greater accuracy than the classic CSF laboratory parameters, sterile CSF pleocytosis due to UTI from that of bacterial and viral meningitis and from control subjects without CSF pleocytosis.