In cancer patients with persistent fever and neutropenia, amphotericin B is administered for the early treatment and prevention of the clinically occult invasive fungal infections. The major drawback of the conventional amphotericin B is its nephrotoxicity. We have previously showed that massive potassium, sodium, and magnesium supplementation, which corresponds to the amount lost in kidney during amphotericin B administration, as well as vigorous hydration, can markedly reduce nephrotoxicity of conventional amphotericin B. Clinical courses of four oncological patients treated with conventional amphotericin B suspected or proven fungal infection for a period of 24 days (two patients), 39 days, and 47 days are described. During the long-term amphotericin B administration with nephroprotective measures, no severe renal function decrease was observed in any of our patients. Massive ion supplementation corresponding to the amounts lost in kidney, as well as the vigorous hydration are effective in the prevention of the renal function decrease induced by the long-term conventional amphotericin B therapy.