Three nonsplenectomized patients were infected with Babesia microti. One had fever, abdominal pain suggesting gallbladder disease, and evidence of disseminated intravascular coagulation; another was considered to have lymphoma, partly because two smears for Babesia before admission were negative. All three patients were treated with pentamidine isethionate and improved clinically. Parasites were no longer seen on smears after 5 days of therapy, but Babesia could still be recovered by hamster inoculation 5 weeks after therapy in one of the patients tested, underscoring the need for this test to properly evaluate eradication of the organism. In one patient, pentamidine was stopped after 7 days because of increased creatinine concentration, and this amount of drug appeared adequate to control the parasitemia. Pain at drug injection sites was a major side effect in all three patients. Pentamidine appears to be useful in controlling clinical manifestations of babesiosis and decreasing parasitemia, but it does not eradicate the organism.