To determine whether a unique group of clinical and laboratory manifestations characterize certain major deer tick-transmitted human pathogens in North America, we compared the symptoms, short-term complications, and laboratory test results of New England residents who became ill due to > or =1 of these pathogens. Patients completed a uniformly structured questionnaire and submitted blood samples for serologic and polymerase chain reaction (PCR) testing after developing symptoms of Lyme disease, human babesiosis, or human granulocytic ehrlichiosis (HGE). Complete blood count with thin blood smear, PCR, and immunoglobulin M antibody tests helped differentiate the acute manifestations of these diseases. Physicians should consider use of tests designed to diagnose babesiosis and HGE in patients with Lyme disease who experience a prolonged flulike illness that fails to respond to appropriate antiborrelial therapy.