Although calculation of the cerebrospinal fluid to serum glucose ratio is widely recommended as a way to identify pathologic hypoglycorrhachia, few data are available to document its accuracy. In order to provide a better basis for interpretation of this quotient, simultaneous cerebrospinal fluid and serum glucose concentrations from patients with diabetes mellitus and noninflammatory cerebrospinal fluid and patients with acute bacterial meningitis were compared. Cerebrospinal fluid to serum glucose ratios were significantly lower in the patients with meningitis (Mann-Whitney U Test, p less than 0.001). A ratio of 0.31 provided the best differentiation between the two groups. Ratios were below this level in 25 of 64 patients with meningitis, including 10 in whom the absolute cerebrospinal fluid glucose concentration was not below 40 mg/dl. In 35 of 36 uninfected diabetic subjects, ratios were 0.31 or greater. In the sole exception, concentrated glucose solution had been given intravenously shortly before lumbar puncture, Use of the cerebrospinal fluid to serum ratio, in addition to the absolute cerebrospinal fluid glucose concentration, increases sensitivity in detecting pathologic hypoglycorrhachia with little loss in specificity.