Fatigue is a major complaint among advanced cancer patients. Several instruments are available for measuring fatigue. The EORTC QLQ-C30 is one of the most frequently used health-related quality of life (HRQOL) instruments, and it includes a three-item fatigue subscale. Limited knowledge exists about the validity, performance and sensitivity of EORTC QLQ-C30 fatigue scale as compared with a fatigue-specific instrument. The aim of the present study was to validate the EORTC QLQ-C30 fatigue scale (FA) against the Fatigue Questionnaire (FQ). The FQ is frequently used and was developed to measure fatigue in both cancer and noncancer populations. The FQ measures physical (PF, seven items) and mental fatigue (MF, four items). The study population included two different cohorts: A) patients with advanced metastatic cancer included in a prospective randomized study of palliative radiotherapy (n = 238); B) patients with leukaemia and malignant lymphoma curatively treated with stem-cell transplantation and high-dose chemotherapy (n = 128). The analysis demonstrated that the FA correlated higher with the PF scale (r = 0.67-0.75) as compared with the MF scale (r = 0.49-0.61). The item scale correlations between FA items and the PF scale were consistently higher than between FA items and the MF scale. A factor analysis including all the items within the FA and the FQ identified two factors. All FA items loaded on a PF factor (0.70-0.85). A floor/ ceiling effect, indicating a high number of respondents with lowest, respectively, highest scores was observed more frequently in the FA as compared with the FQ. The PF discriminated better between diagnostic groups with different levels of fatigue than the FA did. In conclusion, the EORTC QLQ-C30 fatigue scale is measuring physical fatigue. A floor/ ceiling effect seems to appear for the EORTC QLQ-C30 fatigue scale. The validity of the EORTC QLQ-C30 fatigue scale is to be questioned for use in palliative care patients. In studies with fatigue as a defined end point, a domain-specific instrument should, therefore, be added.