A prevalence study of past Lyme borreliosis in persons with outdoor occupations was done. Consenting individuals (n= 302) were administered a questionnaire eliciting demographic and occupational data and a clinical history, and were asked to donate a serum specimen for detection of antibodies toBorrelia burgdorferiby enzyme-linked immunosorbent assay (ELISA), immunoblotting, and borrelia inhibition assays, and for detection of potentially cross-reactive antibodies. Of 302 individuals, 77 (25%) had reactive antibodies detected by ELISA. Of these 302 individuals, 44 (15%) met the criteria of the Centers for Disease Control and Prevention for serological reactivity as evidenced by immunoblotting, and 70 (23%) had inhibitory activity. Through the clinical criteria employed, only 11 individuals with serological reactivity had prior illness compatible with Lyme borreliosis. Higher ELISA absorbances were positively correlated with age and duration of outdoor occupation. The results from three serological assays and the lack of reactivity to potentially cross-reactive infectious agents indicate that serological reactivity was due to exposure toB. burgdorferi.The disparity between serological reactivity and the clinical evidence of Lyme borreliosis suggests cumulative exposure to a nonpathogenic form ofB. burgdorferi.