Although fatigue is the most common symptom reported by cancer patients and has serious adverse effects on quality of life, it remains poorly understood. A survey was designed to characterize the epidemiology of cancer-related fatigue from the perspectives of the patient, primary caregiver, and oncologist. A telephone survey included 419 cancer patients recruited from 100,000 randomly selected households nationwide. Patients provided access to 200 primary caregivers (usually family members) who were also interviewed by telephone. In a separate mail survey, 197 of 600 randomly sampled oncologists (unrelated to the patients) responded to a questionnaire that assessed perceptions and attitudes concerning fatigue in cancer patients who had received chemotherapy or radiotherapy and their caregivers. The median patient age was 65 years, and the principal cancer diagnoses were breast (females) and genitourinary (males). Fifty-nine percent of the patients had received chemotherapy, 63% radiation therapy, and 24% both; 20% of patients received their last treatment within 6 weeks, 31% within 7 weeks to 1 year, and 49% more than 1 year ago. More than three quarters of patients (78%) experienced fatigue (defined as a general feeling of debilitating tiredness or loss of energy) during the course of their disease and treatment. Thirty-two percent experienced fatigue daily, and 32% reported fatigue significantly affected their daily routines. Caregivers reported observing fatigue in 86% of the index patients, and oncologists perceived that 76% of their patients experienced fatigue. Although oncologists believed that pain adversely affected their patients to a greater degree than fatigue (61% v 37%), patients felt that fatigue adversely affected their daily lives more than pain (61% v 19%). Most oncologists (80%) believed fatigue is overlooked or undertreated, and most patients (74%) considered fatigue a symptom to be endured. Fifty percent of patients did not discuss treatment options with their oncologists, and only 27% reported that their oncologists recommended any treatment for fatigue. When used, treatments for fatigue were generally perceived by patients and caregivers to be successful. These data confirm the high prevalence and adverse impact of cancer-related fatigue, although it is seldom discussed and infrequently treated. For patients and oncologists, improving the quality of life of cancer patients requires a heightened awareness of fatigue, a better understanding of its impact, and improve communication and familiarity with interventions that can reduce its debilitating effects.