Few studies have assessed the accuracy and completeness of Lyme disease surveillance systems. Lyme disease cases were identified through review of medical records for residents of the Marshfield Epidemiologic Study Area (MESA), a population-based cohort of nearly 80,000 in north-central Wisconsin for which comprehensive medical care data are available through the Marshfield Clinic. These cases were compared with cases reported to the Wisconsin Division of Public Health to estimate the completeness of reporting. Annual incidence rates were calculated for MESA using the cases identified from chart review. Division of Public Health data were used to calculate statewide reported incidence rates, as well as rates for an eight-county region that surrounds and includes the MESA region. Of Lyme disease cases meeting the national case definition identified in MESA, 34% were reported to the state. The average incidence of Lyme disease (1992-1998) was 19.1 per 100,000 per year in MESA, 17.0 in the surrounding eight-county region, and 9.0 statewide. Trends in reported incidence across time, gender, and age in the surrounding eight-county region were generally comparable with those observed in MESA. These findings suggest that the passive surveillance system monitored trends in Lyme disease incidence reasonably well despite underreporting of cases.