The aim of the present study was to describe fatigue in a group of irradiated cancer patients. Fatigue was assessed prospectively in 81 Swedish cancer patients, using the Multidimensional Fatigue Inventory (MFI-20), which was also validated in the study, the Karolinska Sleepiness Scale and a Borg CR10 scale before curative radiotherapy, at the end of therapy and 1 and 3 months after treatment. The mean age of these patients was 56 years; 90% were female; and 65% were being treated for cancer of the breast. The absorbed radiation dose was 50 Gy or lower in 90% of the patients. A clear peak in experienced fatigue according to the three scales was seen at the end of treatment. In the MFI-20 the dimensions of General fatigue, Physical fatigue and Reduced activity, were more pronounced than the aspects of Mental fatigue and Reduced motivation. Comparison with other studies using the same instrument in other irradiated patients revealed a similar pattern. The internal consistency of the Swedish version of the MFI-20 was good. The MFI-20 is shown to be a valid instrument for assessing fatigue in patients undergoing radiotherapy.