Because inflammation could affect lysosomal enzyme trafficking, resulting in increased enzyme release from the cells, tissue necrosis, or altered blood- and the brain-cerebrospinal fluid (CSF) barrier, the activity of four lysosomal enzymes in the cell-free CSF of 34 patients with bacterial meningitis, 20 with aseptic meningitis, and 39 control subjects was measured. Activities are expressed in nanomoles of 4-methylumbelliferone mL/h. The median beta-hexosaminidase A activity in bacterial meningitis was 313, in aseptic meningitis it was 173, and in the control subjects it was 175, the median beta-hexosaminidase B activity was 417, 165, and 120; the median alpha-mannosidase activity was 171, 124, and 113; and the median beta-glucuronidase activity was 133.7, 14.3, and 10.0, respectively. The difference of the activities of the four enzymes measured between the bacteria meningitis and the controls is significant (p < 0.000). Also significant is the difference between bacterial and aseptic meningitis (p = 0.005 to < 0.000), but it is not significant between aseptic and control subjects. Both the sensitivity and specificity of the beta-glucuronidase activity between bacterial meningitis and control subjects were 100%, whereas the corresponding values between bacterial and aseptic meningitis were 100% and 90%, respectively. No significant correlation was observed between the activities of the enzymes measured and the number of the polymorphonuclear leukocytes or other laboratory characteristics of the CSF. The increased lysosomal enzyme activities in the CSF of patients with meningitis may result from diffusion across the blood-CSF or the brain-CSF barrier or from enzyme leakage through the cell membranes.