Fatigue has been recognized as the most frequently reported symptom of cancer and cancer therapy. There is a lack of research on such aspects as the pattern of fatigue which accompanies treatment, its exacerbating and relieving factors, the different mechanisms of fatigue and its relationship with the factors purported to be related to the fatigue experience. It has been suggested that cancer patients may be those best placed to reveal the likely causes of fatigue. As part of a larger study examining the pattern of fatigue in cancer patients who were undergoing a course of chemotherapy treatment, patients' perceptions of fatigue and tiredness and the nature, pattern and causes of fatigue in relation to cancer and its treatment were obtained. In addition to a diary, interviews were conducted at two time points, at the beginning and end of a cycle of chemotherapy, with over 100 cancer patients. Just under 90% of the sample reported fatigue at some point during a cycle of chemotherapy. The majority of the sample did not consider tiredness and fatigue to constitute the same feelings. Subjects attributed their fatigue to a combination of factors but most frequently mentioned treatment, changes in sleep patterns and other symptoms. Implications for practice and research are outlined.