Fever is associated with malignancy and is a common problem in cancer patients. Fever in the cancer patients is closely linked with infection, especially when the patient is granulocytopenic. When fever appears, a series of diagnostic and therapeutic measures must be taken even if precise knowledge of the cause of the infection is lacking. Fever can be caused by infection or by the cancer itself through tumor-related necrosis, hemorrhage or pyrogens. Infection is the more common cause, however. Bacterial and fungal sepsis can coexist and the bacteremia can overshadow the more difficult to determine fungal infection. For this reason it has become accepted practice to administer amphotericin B to granulocytopenic patients who remain febrile after a few days of broad-spectrum antimicrobial therapy and in whom no bacteria can be documented. Viral infection is rarely diagnosed in neutropenic patients without concomitant immunosuppression.