This study compared the severity of fatigue in patients with cancer to the fatigue reported by depressed psychiatric patients and community-dwelling adults. Data were collected for this study during the process of validating a new fatigue assessment tool, the Brief Fatigue Inventory (BFI). The sample included 354 cancer patients, 72 psychiatric patients, and 290 non-patient volunteers. Study subjects reported severity of fatigue and the degree to which fatigue interfered with various aspects of life. Data were also collected on sleep disturbance and demographic variables that might correlate with fatigue. The psychiatric patients reported significantly higher levels of fatigue and fatigue-related interference than the cancer patients, who reported more severe fatigue and interference than the community subjects. The sleep disturbance scores of the cancer patients and the community subjects were significantly correlated with fatigue severity. Although the majority of the psychiatric patients reported sleep disturbance, their sleep disturbance scores were not significantly associated with fatigue severity.