The aim of this work was to reinvestiagte usual values of neurotransmitter catabolites (5-HIAA and HVA) in CSF, their determination being a current method for studying pathological processes and/or drug induced changes on CNS metabolism. 5-HIAA and HVA have been measured by reverse phase HPLC and electrochemical detection. Since many confounding factors have been identified to modify the concentration of both analytes in CSF, data from 1.480 patients have been revisited with strict exclusion and 323 have been selected. Age dependent variation and asymetric distributions were observed. Age-related usual values ranges were established after identification of outliers by non-parametric analysis. In a group of patients for which repeated CSF collection have been performed for the follow up of acute leukemia, wide and unpredictible individual variations (up to 400 %) of CSF 5-HIAA and HVA concentrations have been observed. These large interindividual and individual variations may explain the numerous discrepancies found in studies investigating the CNS monoamine metabolism, especially when small groups of patients are used. Such observation limits the interpretation of monoamine metabolic for diagnostic purposes without a strict selection of indication.