We sought evidence of babesiosis in three residents of New Jersey who were suspected of local acquisition of Babesia microti infection. We tested serial blood samples from these residents for B. microti antibodies and amplifiable DNA by using immunofluorescent antibody and PCR techniques. All three residents experienced symptoms suggestive of acute babesiosis. The sera of each of the patients reacted against babesial antigen at a titer fourfold or higher in sequentially collected blood samples. PCR-amplifiable DNA, characteristic of B. microti, was detected in their blood. These data suggest that human B. microti infections were acquired recently in New Jersey, extending the range of this piroplasmosis in the northeastern United States.