Fatigue is one of the most common and debilitating symptoms experienced by cancer patients, yet until recent years it has received little systematic attention, due in part to the lack of adequate instruments to measure fatigue. The primary aim of this report is to further validate a recently developed measure of fatigue for use with cancer patients: the Fatigue Symptom Inventory (FSI). This 13-item self-report measure was designed to measure the intensity and duration of fatigue and its interference with quality of life. The FSI was originally validated in a sample of breast cancer patients and a sample of healthy individuals. In this study, the FSI was evaluated in an outpatient sample that included male and female cancer patients, as well as some older patients, with a variety of cancer diagnoses. A seven-item interference scale was found to have good internal consistency, with alpha coefficients above 0.90. Convergent validity was demonstrated via comparisons with an existing measure of fatigue. Construct validity was demonstrated via comparisons with measure of life satisfaction and depression as well as comparisons among subgroups of patients expected to differ in their experience of fatigue. Overall, the FSI was further established as a valid and reliable measure of fatigue in cancer patients. The potential application of this measure in psychosocial oncology research is discussed.