The diagnosis of central nervous system infection, especially bacterial meningitis, remains the most compelling reason for lumbar puncture and examination of the cerebrospinal fluid. The traditional tests performed include the opening pressure, the protein and glucose concentrations, the total cell count and differential, and cultures of microbiological agents. These tests are obtained in virtually all cases of suspected central nervous system infection. The routine cerebrospinal fluid profile of cell count, protein, and glucose is an important guide to the diagnosis of noninfectious central nervous system disorders as well, including those of neoplastic, vascular, autoimmune, and degenerative etiologies. The routine availability of computed tomographic (CT) scanning and other imaging techniques has made it possible to visualize mass lesions rapidly and avoid lumbar puncture in many patients with increased intracranial pressure and risk of cerebral herniation.A number of newer tests of cerebrospinal fluid components beyond the routine profile are presently available, and others are being actively developed. A long list of enzymes, antigens, and ionic components can be determined on cerebrospinal fluid. In multiple sclerosis, studies of IgG and oligo clonal bands are now a major diagnostic aid. IgG studies are also of value in patients with autoimmune disease and chronic inflammatory and infectious disorders. In neoplastic disorders, a limited number of tumor markers and antigens may be presently measured on cerebrospinal fluid to guide diagnosis and treatment. The most important of the newer cerebrospinal fluid tests are those