Infection remains a major complication in patients with malignant disease. There are many factors predisposing to infection in this patient population, including local factors due to the tumor, specific deficiencies in host defense mechanisms due to certain malignant processes, and deficiencies in host defense mechanisms secondary to cancer chemotherapy. Neutropenia is probably the most important factor predisposing to infection in cancer patients. These patients require prompt broad-spectrum antibiotic therapy when fever develops. The majority of infections occurring in this patient population are caused by gram-negative bacilli and cure rates usually are between 65 and 75 percent. The most important prognostic factor is whether or not the neutrophil count recovers during the course of infection. Fungal infections have increased in frequency in neutropenic patients and often present as fevers of unknown origin. Increasingly, neutropenic patients are receiving antifungal agents as empiric therapy for persistent fever that fails to respond to antibacterial antibiotics. The most critical factor in recovery from fungal infection is remission of the underlying malignant disease.