White and red blood cell counts from peripheral blood and from the cerebrospinal fluid of all patients who had traumatic lumbar punctures over a five-year period were studied to determine the diagnostic value of using the ratio of white blood cells to red blood cells in peripheral blood to adjust the white blood cell count found in the bloody cerebro-spinal fluid. In patients without meningitis, 55 percent of the cerebrospinal fluid specimens had more white blood cells than could be attributed to trauma. However, in only 10 percent was the actual white blood cell count more than 10 times greater than the number referable to trauma. In 38 percent, there were fewer white blood cells in the cerebrospinal fluid than the ratio predicted. Such underestimation was uncommon among patients with culture-positive meningitis: the ratio failed to detect leukocytosis in only 10 percent, and all but one of these patients had clinical reasons for the low cerebrospinal fluid count. It is concluded that true leukocytosis is rarely masked in blood-contaminated cerebrospinal fluid, and that the presence of more than 10 times the number of white blood cells than allowed by the adjustment is a sensitive and specific indicator of meningitis. However, a simple mechanical use of the formula is not justified, and all available clinical and laboratory data should be used when deciding whether to institute treatment in a patient with possible meningitis and bloody spinal fluid.