The field of opportunistic mycoses in the patient with cancer is rapidly changing. Not only are fungal infections increasing in frequency in this patient population, but these infections are occurring earlier during the course of cytotoxic chemotherapy, and newer fungi are increasingly recognized as potentially lethal pathogens. Candidiasis remains the most commonly encountered infection. The spectrum of disease includes candidemia and acute and chronic disseminated candidiasis. Pulmonary aspergillosis and disseminated aspergillosis are common and remain relatively resistant to therapy. Disseminated fusariosis and trichosporosis are almost always fatal in the setting of persistent profound neutropenia. Therapy for these mycoses relies on the use of amphotericin B and 5-fluorocytosine. Newer antifungal agents, such as fluconazole and itraconazole, appear to exhibit good activity against a variety of fungi. Newer approaches need to be tested for the treatment of the more-resistant mycoses and may include the use of maximally tolerated doses of antifungal agents, colony-stimulating factors, and combination therapy.