The aim of this investigation was to estimate the seasonal risk of contracting human Lyme disease in different vegetation types in southern Sweden. Host-seeking Ixodes ricinus ticks (Acari: Ixodidae) were collected with standardized methods during May-September 1988 and March-October 1989 at 10 different sampling sites. Tick abundance was greatest during May-June and August-September. Prevalence of Borrelia burgdorferi infection of the ticks was assessed by phase-contrast microscopy complemented by immunology. Spirochetal infection of tick larvae was not detected. The prevalence of infection among nymphal ticks differed significantly between years and between sampling sites. Infection prevalence was greater in adult females than in nymphs, but was similar in female and male ticks. Among all vegetation types studied, the greatest Lyme disease risk was deemed to be from I. ricinus nymphs during May and September in mixed forest vegetation.