The aim of the study was evaluation of usefulness of cerebrospinal fluid (CSF) lactate dehydrogenase (LDH) activity assessment in diagnostics of purulent, bacterial meningoencephalitis in adults. The investigations were performed in 17 subjects. In all individuals CSF and plasma LDH activity was estimated during the first 24 hours of hospitalization. Mean CSF LDH activity in patients in very severe clinical state (group I) was 299,11 U/L compared to 163,67 U/L in subjects of group II with moderate and mild course of disease. The difference between mean CSF activities of this enzyme was statistically significant (p<0,001). The obtained results indicate the usefulness of CSF LDH activity assessment in estimation of severity of the patient's clinical state. The magnitude of this activity seems to be also helpful as prognostic marker in purulent, bacterial meningoencephalitis.