The distributional area of the tick Ixodes ricinus (L.), the primary European vector to humans of Lyme borreliosis spirochaetes (Borrelia burgdorferi sensu lato) and tick-borne encephalitis virus, appears to be increasing in Sweden. It is therefore important to determine which environmental factors are most useful to assess risk of human exposure to this tick and its associated pathogens. The geographical distribution of I. ricinus in Sweden was analyzed with respect to vegetation zones and climate. The northern limit of I. ricinus and B. burgdorferi s.l. in Sweden corresponds roughly to the northern limit of the southern boreal vegetation zone, and is characterized climatically by a mean duration of 150 days with snow cover and a vegetation period averaging 170 days. The zoogeographical distribution of I. ricinus in Sweden can be classified as southerly-central, with the center of the distribution south Limes Norrlandicus. Ixodes ricinus nymphs from 13 localities in different parts of Sweden were examined for presence of B. burgdorferi s.l. and found to be infected with B. afzelii and B. garinii. Tick sampling localities were characterized on the basis of density of Borrelia-infected I. ricinus nymphs, presence of specific mammals, dominant vegetation and climate. Densities of I. ricinus nymphs and Borrelia-infected nymphs were significantly correlated, and nymphal density can thus serve as a general indicator of risk for exposure to Lyme borreliosis spirochaetes. Analysis of data from this and other studies suggest that high densities of Borrelia-infected nymphs typically occur in coastal, broad-leaf vegetation and in mixed deciduous/spruce vegetation in southern Sweden. Ixodes ricinus populations consistently infected with B. burgdorferi s.l. can occur in biotopes with (i) shrews, rodents, hares and birds, (ii) shrews, rodents, hares, deer and birds, (iii) and on islands where the varying hare (Lepus timidus) is the only mammalian tick host.