Thirty-four consecutive patients were hospitalized with diagnosis of severe Babesia infection over the course of 13 years. The average time from onset of symptoms to diagnosis was 15 days. When compared with uninfected febrile control patients, affected patients complained significantly more often of malaise, arthralgias and myalgias, and shortness of breath (P<.05), and they more often had thrombocytopenia and abnormal liver function (P<.05). Forty-one percent of patients with Babesia developed complications such as acute respiratory failure, disseminated intravascular coagulation, congestive heart failure, and renal failure. Analysis of data revealed that complicated babesiosis was more commonly associated with the presence of severe anemia (hemoglobin level <10 g/dL; P=.01) and higher parasitemia levels (>10%; P=.08). Patients were treated with a combination of drugs that included clindamycin, quinine, atovaquone, or azithromycin. Despite treatment, parasitemia persisted for an average of 8.5 days (range, 3--21 days). Exchange transfusion was performed for 7 patients, and it effectively reduced the high levels of parasitemia. Three patients died. Improved outcomes may result with prompt recognition and treatment of babesiosis.