Epidemiological data on 136 cases of human babesiosis reported from laboratories and clinicians in the state of New York from 1982 to 1991 were reviewed. All but two patients, who had traveled to Nantucket Island in Massachusetts, acquired disease in Suffolk County, Long Island. The highest average age-group-specific annual incidence rates occurred among men > or = 80 years of age (7.72 per 100,000) and among women 70-79 years of age (3.61 per 100,000). Seven patients (5%) had previously undergone splenectomy, and 31 (23%) had evidence of concurrent Lyme disease. One hundred three patients (76%) were hospitalized, and seven (5%) died. Although the geographic distribution of this disease has remained constant, the recent introduction of babesiosis in mainland Connecticut and the spread of Lyme disease in New York suggest that the geographic distribution of babesiosis could also spread in New York.