During the 20-month study period, which covered two summers of Lyme disease transmission, 1917 of the 10 936 study participants were evaluated for possible Lyme disease (19). Of the 1917 patients, 146 (7.6%) met study criteria for definite Lyme disease, and 118 (6.2%) had microbiological confirmation of this infection by culture or PCR testing of erythema migrans. The mean age of these 118 patients was 51 years (range, 17 to 71 years); 53% were men, and 47% were women. Forty-seven percent were from New England, 51% were from mid-Atlantic states, and 2% were from Wisconsin, reflecting the locations of the study sites. June and July were the peak months of disease onset, which correlated with the expected peak questing period of nymphal Ixodes scapularis ticks. However, cases occurred from March through October, suggesting that adult ticks may also transmit the disease. Vaccine and placebo recipients did not differ in the size of erythema migrans, persistence of symptoms after treatment, and morphologic characteristics of the lesions. In addition, no clinical differences were noted in different geographic areas. Therefore, we present data from vaccine and placebo recipients and from different geographic areas together.