555 consecutive cases in which cerebrospinal fluid (CSF) was sent for cell count were reviewed to determine which cerebrospinal-fluid tests affect diagnosis or therapy. Among 334 cases (60%) with a normal opening pressure, cell count, and protein, 1385 additional tests were done, but such tests were useful in only 3 patients (0.9%) with multiple sclerosis. Among 148 consecutive cases of bacterial, chronic infectious, and malignant meningitis the opening pressure, cell count, or protein was abnormal in all but 3 (2 childhood bacterial meningitis and 1 cryptococcal meningitis in a patient with the acquired immunodeficiency syndrome). If the opening pressure, cell count, and protein are normal, no additional CSF tests are needed in most instances; however, in immunocompromised patients and in those with possible multiple sclerosis or childhood bacterial meningitis additional tests may be indicated.