Between 1969 and 1977, 14 persons with parasitologically confirmed Babesia microti infections and seven persons with antibody titers to B. microti greater than or equal to 1:1,024 were identified on Nantucket Island, Massachusetts. Nineteen of these 21 persons were interviewed. About half were permanent residents of Nantucket; the others spent most of their summers on the island. There were 12 women and seven men. Patients ranged in age from 23 to 86 years; all of those with parasitologically confirmed infections were at least 49 years old. Fifteen patients had illnesses characterized by fever, chills, myalgia and fatigue. Five reported being bitten by a tick from 7 to 28 days before the onset of illness. Most cases occurred during July or August. There appeared to be no association between B. microti infection and direct contact with wild or domestic animals or specific outdoor activities. The unusual age distribution of patients with parasitologically confirmed B. microti infections may result because older persons tend to have more severe illnesses and thus are more likely to come to medical attention.