To determine the occurrence of cancer-related fatigue, the methods used to assess it, and the efficacy of the available treatments, we performed literature searches that identified English-language publications on these topics. Twenty-seven studies were identified in which the quantitative estimation of the occurrence of cancer-related fatigue was an end point. Fifty-six were judged to be relevant to the assessment of fatigue, and 10 randomized controlled clinical trials of treatments of cancer-related fatigue were retrieved. The occurrence of cancer-related fatigue was found to range from 4% to 91%, depending on the population studied and the methods of assessment. Few population-based studies and no longitudinal studies of cancer-related fatigue have been performed. The methods of fatigue assessment were highly variable. Exercise programs show promise to prevent or treat fatigue in some subsets of cancer patients, and the use of epoetin alfa for correction of anemia has been shown to ameliorate fatigue. The number of subjects in the treatment trials was small and their methodologic quality was inconsistent.