To evaluate the proposed cancer-related fatigue (CRF) diagnostic criteria in a sample of cancer survivors. More accurate prevalence estimates of CRF may result in improved diagnosis and management of one of the most common symptoms associated with cancer and its treatment. Three hundred seventy-nine individuals who had been treated with chemotherapy, either alone or in combination with radiation therapy, were surveyed. Patients were asked background questions about their current condition, their medical history, and the frequency of fatigue during their chemotherapy. Additionally, patients who reported experiencing fatigue at least a few days each month during treatment were asked a series of questions about the impact of fatigue on their daily functioning. One hundred forty-one (37%) individuals reported at least 2 weeks of fatigue in the previous month. Of the respondents who had received their last treatment more than 5 years ago, 33% still reported at least a 2-week period of fatigue in the month before the interview. Evaluation of the proposed criteria revealed that 17% of respondents met at least two criteria for CRF. The prevalence of diagnosable CRF in the individuals in this sample, most of whom had completed treatment more than 1 year ago, was 17%-lower than expected based on previous reports that have used less-strict criteria. In a sizable number of people, CRF persists well beyond active treatment and should be a focus of intervention. Although they will require replication in other samples and clinical validation, these formal diagnostic criteria can be a step toward common language and a better understanding of the severity range and persistence of CRF.