Objective: To attempt to validate a previously reported clinical prediction rule derived to assist in distinguishing between acute bacterial meningitis and acute viral meningitis. Design: Retrospective chart review of patients treated at five hospitals between 1981 and 1990. The criterion standard for bacterial meningitis was a positive cerebrospinal fluid (CSF) or blood culture or a positive test for bacterial antigen in the CSF. For viral meningitis, the criterion standard was a positive viral culture from CSF, stool, or blood or a discharge diagnosis of viral meningitis with no other etiology evident. Setting: Two Department of Veterans Affairs (VA) hospitals, two county hospitals, and one private hospital, each affiliated with one of two medical schools. Patients: All persons aged more than 17 years who were hospitalized over a ten-year period at one of five academically affiliated hospitals for the management of acute meningitis. Measurements and main results: Sixty-two cases of bacterial meningitis and 98 cases of viral meningitis were confirmed. With all patients included, the discriminatory power of the model as measured by the area under the receiver operating characteristic curve (AUC) was 0.977 (95% CI, 0.957-0.997), compared with the AUC of 0.97 in the derivation set of the original publication. The AUCs (95% CIs) for data subsets were: Dallas cases 0.994 (0.986-1.0), Milwaukee cases 0.912 (0.834-0.990); ages 18-39 years 0.952 (0.892-1.0), ages 40-59 years 0.99 (0.951-1.0), and age >60 years 0955 (0.898-1.0). Conclusions. The authors conclude that the clinical prediction rule proved robust when applied to a geographically distinct population comprised exclusively of adults. There was sustained performance of the model when applied to cases from each city and from three age strata. Prospective validation of this prediction rule will be necessary to confirm its utility in clinical practice. © 1994 Society of General Internal Medicine.