Antibody prevalence and clinical manifestations of Lyme borreliosis (LB) and tick-borne encephalitis (TBE) were studied in 362 orienteers from the county of Stockholm during a large relay race in October 1990. From all participating orienteers, a blood sample was collected and a questionnaire completed. Antibody activity to Borrelia burgdorferi was measured using a sonicated whole spirochete antigen in an ELISA, and to TBE virus (TBEV) by ELISA and haemagglutination inhibition (HI) test. A past history of LB was reported by 6% of the orienteers and antibodies to B. burgdorferi were found in 9%. Antibody reactivity to B. burgdorferi was found in 1-2% of the sera from 3 different control groups comprising 502 persons living in non-endemic areas. The corresponding value was 9% in sera from a fourth control group consisting of 150 persons living in the city of Stockholm. A past history of TBE was reported by 1 orienteer (0.3%) and detectable antibodies to TBEV in non-immunized individuals were found in 1%. No antibody activity to TBEV could be detected in sera from the control individuals living in the 3 non-endemic areas, but in 5% of the controls from Stockholm. In this study, there was no indication that frequent or severe manifestations of LB or TBE are common among orienteers in Sweden.