Catalonia is an endemic area of Mediterranean spotted fever. In 1997, A. Lakos described a new tick-borne infectious disease called tick-borne lymphadenopathy. The causative agent is Rickettsia slovaca, which is transmitted by Dermacentor marginatus ticks. We have diagnosed human cases in Catalonia. The objective of this study was to determinate seroprevalence of R. slovaca infection in humans in the northeast of Spain. The population included 217 subjects from Catalonia, northeast of Spain and was stratified by age and living place (rural, suburban, and urban). Age, gender, residence area, contact with animals, occupation, and history of rickettsioses was surveyed. Immunoglobulin G was measured by indirect immunofluorescence assay. Titers >or= 1/40 were considered. Seroprevalence of R. slovaca was 5.5% at titers of 1/40-1/320. Eight (3.7%) sera had antibodies against R. slovaca exclusively. Four sera reacted also against Rickettsia conorii and/or Bar29. Seroprevalence of R. slovaca would range from 3.7% to 5.5%. The only statistically significant association was that between R. slovaca seropositivity and age. We present serologic evidence of R. slovaca infection among population of Catalonia, northeast of Spain.