Since the treatment of fungal infections with amphotericin B may result in significant nephrotoxicity, better methods for discriminating between life-threatening and more benign fungal infections are needed. Recently numerous fungal casts were identified in the urine of a patient who had undergone renal allograft transplantation. The recognition of fungal casts permitted an unequivocal diagnosis of systemic fungal infection. Successive examinations of the patient's urinary sediment provided an excellent monitor of the response to treatment. The cytologic features of fungal casts are described. Since systemic fungal infections often involve the kidney, screening for fungal casts may have significant clinical applicability.