Serum samples from 127 Dutch forestry workers and 127 matched controls were tested for antibodies against Borrelia burgdorferi in an indirect immunofluorescence assay (IFA). Those of the forestry workers were also tested by Western blotting. The forestry workers were examined clinically for evidence of Lyme borreliosis without the examiner or the workers knowing the results of the laboratory tests. Seroprevalence of B. burgdorferi antibodies among forestry workers (25/127) was significantly higher than among controls matched for age and place of residence (8/127), odds ratio 3.7 (95% CI 1.5-9.7). Of the 25 sera of forestry workers positive in the IFA, 23 reacted with at least five bacterial polypeptides in the Western blot test. According to adapted CDC criteria, seven forestry workers (6%) were classified as being a case of Lyme borreliosis. In only one of them had the diagnosis been made before this investigation. Five persons had a history of erythema migrans, one of arthritis, and one of persistent infection. We conclude that Lyme borreliosis is an occupational disease among forestry workers in the Netherlands, with a three-fold higher seroprevalence than among matched controls. The disease, often not diagnosed among this high-risk group, warrants more attention to achieve early recognition and to prevent late complications.