Biological markers of tumors are compounds or enzymatic activities measurable in body fluids. Their presence or concentration must be linked to tumoral growth. The markers of the central nervous system tumors are detected in CSF. Alpha-feto-protein, carcinoembryonic antigen, human chorionic gonadotropin, adenohypophyseal peptide hormones, enzymes, etc., have found some application in the early diagnosis of leptomeningeal metastasis. Other applications involve the early detection and recurrency of primary brain tumors, as well as the evaluation of efficacy of their therapy. The tests based on the CSF content of desmosterol and polyamines have been studied extensively. Their rationale is discussed and specificity, sensitivity, efficiency and predictive value are considered. Experimental results concerning a new possible biochemical marker, based on CSF concentration of cyclic adenosine monophosphate, are reported.