Lyme disease has been reported in Minnesota since 1980. Until now, it has not been regarded as endemic in Olmsted County. We reviewed the medical records of all 68 cases of Lyme disease diagnosed in residents of Olmsted County for 1980-1990. Only 17 of the 68 cases met Centers for Disease Control surveillance criteria for definite Lyme disease (13 men, 4 women). Of these 17 cases, 3 were likely acquired in Olmsted County. The age and sex adjusted incidence rate was 2.3/100,000 (95% CI 1.1-3.4). Although reporting of Lyme disease is mandatory in Minnesota, only 7 suspected or definite cases were actually reported to public health officials. Of these, 4 met the case definition. In this population Lyme disease was both over-diagnosed and underreported. This observation questions current data and could have important health policy implications if confirmed nationally.