Human babesiosis (Nantucket fever) is a rare, tick-borne intraerythrocytic parasitic disease characterized by fever, lymphadenopathy, arthralgias, and hemolytic anemia. A 34-year-old woman, who had previously undergone surgical removal of her spleen, was hospitalized because of presumed hepatitis. The many retinal nerve fiber layer infarcts and serologic abnormalities suggested collagen disease. The diagnosis of babesiosis was established by the demonstration of intraerythrocytic parasites and a greatly elevated titer to Babesia microti. The infection was treated with chloroquine, orally administered quinine, and pyramethamine, and her symptoms resolved within one month. The retinopathy was probably the result of focal vasculitis secondary to immune complex disease caused by chronic infection.