Among 2403 ticks (Ixodes ricinus) tested in 1985 for Borrelia 328 (13.6%) were carriers (adults about 20%, nymphs about 10%, larvae about 1%). The highest prevalence of infected ticks was among adult ticks in the Isar region north of Munich (33.8%). Among 9383 persons whose serum had been examined by fluorescence serology in 1985 and 1986, 1035 (11%) had raised Borrelia-specific IgG and/or IgM antibodies greater than or equal to 1:64. In 18.7% only IgM antibodies were demonstrated. Among 375 proven cases there were 78 with erythema migrans, 211 with neurological signs, 48 with Lyme arthritis and 36 with acrodermatitis. Seasonal incidence peaks were in June-August for erythema migrans, July-September for neurological signs, with no clear-cut seasonal peaks with Lyme arthritis and acrodermatitis. The incubation time for 80% of cases of each abnormality was 5-29 days for erythema migrans, 20-59 for neurological signs and 2-8 months for Lyme arthritis. Erythema migrans was most frequent among those aged 30-60 years, neurological signs among children and juveniles up to 20 years and those aged 40 to 70 years, Lyme arthritis among those aged 30-60 years, and acrodermatitis among those aged 40-80 years. Significantly more women than men developed acrodermatitis.