From September 1992 to August 1993, 338 patients over the age of 15 years presented to the Department of Infectious Diseases, University Medical Centre Ljubljana, with acute lymphocytic meningitis. In 89 of these patients (26.3%) serum IgM and IgG antibodies against tick-borne encephalitis (TBE) virus were detected, and in 59 patients (17.5%) a borrelial etiology of disease was demonstrated by one or more of the following presence of intrathecal antibody production, seroconversion to borrelial antigens, presence of erythema migrans, and/or isolation of Borrelia burgdorferi sensu lato from skin or cerebrospinal fluid. Of the 148 patients who fulfilled criteria for TBE or borrelial infection, concomitant infection with TBE virus and B. burgdorferi sensu lato was demonstrated in 12 patients (3.6% of all patients presenting with acute lymphocytic meningitis). In the majority of patients with concomitant infection the clinical features at presentation were characteristic of, or consistent with, TBE. In addition, during follow-up studies, eight of the 12 patients subsequently developed signs and symptoms compatible with minor and/or major manifestations of Lyme borreliosis. Six patients were diagnoses with neuroborreliosis based on signs or symptoms and/or laboratory tests. These findings show that in patients with acute lymphocytic meningitis or meningoencephalitis, originating in TBE and Lyme borreliosis endemic regions, the possibility of concomitant infection should be considered.