A splenectomized woman with a history of hepatic disorders was diagnosed as having babesiosis. The patient was unsuccessfully treated with chloroquine and with pentamidine isothionate. A parasitemia of 15 per cent was reduced permanently to less than 1 per cent after a red blood cell exchange, but a low grade parasitemia still existed 10 months after onset. On two separate occasions, the patient was found to have selective IgA deficiency, a reduction of T lymphocytes, and a reduction in function of both T and B lymphocytes. This case represents the highest and the longest duration of parasitemia ever recorded. It reports the first use of pentamidine and red blood cell exchange transfusion in human babesiosis, one of the earliest diagnosed cases of babesiosis, and the most severe clinical case to survive.