Studies on cerebrospinal fluid (CSF) concentrations of neuron-specific enolase (NSE), S-100 protein, and myelin basic protein (MBP) in patients with neurological lesions indicate a quantitative relation between the degree of cell damage in the central nervous system (CNS) and the concentration of these CNS-specific proteins in CSF. Thus NSE, S-100, and MBP could be of use as markers for destructive processes in the CNS. We collected 937 specimens of CSF from children and adults (from newborns to age 91 years) who were undergoing a diagnostic lumbar puncture for several clinical indications. Of these, 79 samples from subjects ranging in age from 0.7 to 66 years could be used retrospectively to construct a reference interval according to our criteria. In these 79 samples no sex dependency existed. The relative increase of NSE, S-100, and MBP with age was similar (1% per year), suggesting a common underlying mechanism. These results emphasize the necessity of using age-matched reference values when the CNS-specific proteins are to be evaluated in neurological diseases. We also present three case histories to discuss the possible clinical relevance of the measurement of NSE, S-100, and MBP in children and adults.