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ABSTRACTPrevious studies from the late 1980s defined the risk of human Lyme disease by determining the prevalence of Borrelia burgdorferiinfection in Ixodes scapularisticks and Peromyscussp. mice captured from areas around La Crosse, Wis. High percentages of B. burgdorferi-infected I. scapularisticks and P. leucopusmice were common in areas located north of Interstate 90 but were not detected in areas south of this major east-west thoroughfare. In this study, we reevaluated the extent of B. burgdorferiinfection. High percentages of mice captured from sites north of the interstate were still infected with B. burgdorferi. In addition, B. burgdorferiwas recovered from 12 (67%) of 18 mice captured from a site well south of the highway. However, none of 104 mice or 713 I. scapularisticks captured from the study sites were infected with Ehrlichiaspp. The results confirmed the continued high risk for humans to contract infection with B. burgdorferiand the significant southward expansion of the area in which Lyme disease is endemic. In contrast, the risk of acquiring human granulocytic ehrlichiosis remains minimal despite the abundance of appropriate vector ticks and reservoir rodents.