The Multidimensional Fatigue Inventory (MFI-20) scale is widely used for measuring fatigue in cancer care. This questionnaire has been translated into Swedish and used in Swedish cancer populations, and the aim of this study was to test the validity and reliability of the Swedish version in four populations, with a total of 584 patients. The participants were classified into four groups: palliative cancer patients, cancer patients receiving radiation therapy, non-cancer outpatients, and a group of hospital staff. The MFI-20 consists of five subscales of fatigue: General Fatigue (GF), Physical Fatigue (PF), Reduced Motivation (RM), Reduced Activity (RA) and Mental Fatigue (MF). We have tested the convergent validity of the MFI-20 using the Category Ratio instrument (CR-10). The validity and the reliability of MFI-20 were acceptable. All subscales of the MFI-20 were correlated, and all were also correlated with the CR-10 score (p < or = 0.001). General Fatigue was highly correlated with Physical Fatigue for the three patient groups, but this was not the fact for healthy staff. Deleting some items increased Cronbach's alpha of the subscale to which these items belonged (where alpha measures the reliability of the results). The level of non-response was low (less than 1.2%) and there was no pattern to the items omitted. We conclude that the MFI-20 is a valid and reliable instrument for measuring fatigue in patients and in healthy individuals. The results support, to some extent, earlier findings and one item can be removed from the Swedish version of the MFI-20.