The hard tick Ixodes ricinus, vector of Borrelia burgdorferi and TBE virus (TBEV), is most abundant in the southern and central parts of Sweden. About 2000 cases of Lyme borreliosis (LB) and 50-80 cases of tick-borne encephalitis (TBE) are estimated to occur in Sweden annually. Five populations, including 903 individuals living in five different areas close to Stockholm where LB and TBE are endemic, were studied with regard to the clinical manifestations and antibody prevalence of LB and TBE. The study areas involved four groups of islands in the Baltic Sea and one island in Lake Mälaren. A history of LB was reported by 1-21% of the participants and antibodies to Borrelia burgdorferi were found in 7-29% of the individuals from the various areas. An increasing seroprevalence with age was seen. A history of TBE was reported by 0-6% of the individuals and in non-immunised participants seropositivity was seen in 4-22%, depending on the area investigated. The individuals from one of the areas, where 30% of Ixodes ricinus ticks had been found to be infected with Borrelia spirochetes, were followed during a two-year period. In the first year of the study, 4.6% of the subjects (n = 303) developed LB and in the second year 3.2% (n = 277). A significant increase in the antibody titre for Borrelia burgdorferi was seen in 4.6% of the individuals in the first year and in 2.9% in the second year. In the first year of the study, 0.3% of the subjects developed TBE and in the second year 0.4%. Seroconversion for TBEV was found in 1.2% of the subjects in the first year and in 2.4% in the second year. In 362 orienteers from the county of Stockholm, a past history of LB was reported by 6% of the individuals and 9% of them were seropositive. A past history of TBE was reported by 0.3% of the orienteers and 1% of the individuals were seropositive. A total of 3141 Ixodes ricinus ticks, 2740 adults and 401 nymphs, were collected from different localities in 23 of the 25 provinces in Sweden. The ticks were examined for the presence of Borrelia spirochetes by indirect immunofluorescence. The prevalence of Borrelia-infected Ixodes ricinus ticks varied from 10-20% in the southern and central parts of Sweden to about 5% in the northern part (Norrland). Of 41 non-Ixodes ricinus ticks, none was positive to Borrelia.