Fatigue is a common and distressing complaint of cancer patients. It is typically measured with symptom inventories that reflect the patient's experience over the previous days or weeks. This study examined short-term variation in fatigue levels in a heterogeneous group of cancer patients over a 3-day period to examine the feasibility of such repeated assessments and to characterize the extent and pattern of fatigue symptoms in cancer patients. Thirty-four cancer outpatients with diverse malignancies wore a prototype fatigue watch monitor for three consecutive 24-hour periods and provided fatigue ratings every hour while awake for the 3 days. Patients completed an average of 40 self-reports over 72 hours. These reports revealed a diurnal variation in fatigue, with increasing levels in the evening. The reports also revealed considerable differences across individuals and within individuals in terms of fatigue ratings. Multiple ratings of fatigue within short periods of time can be obtained and reveal that fatigue levels are quite variable, even within an individual. Cancer patients experience their fatigue as "moderate to extreme" 33% of the time.