Fatigue has become an important symptom in research and also for clinical diagnosis and follow up. Many physical illnesses, in particular chronic ones such as cancer, are highly associated with fatigue. Various questionnaires for measuring fatigue have been developed, but currently no validated questionnaire exists in French language. We selected the ‘Multidimensional Fatigue Inventory’ (MFI) which has been validated in its English version and then translated into French by the designers. This study describes the validation step of the French version of MFI and presents the psychometric properties of this instrument. A sample of 225 patients was divided into three groups ‘Tired’ (82 subjects), ‘Moderately tired’ (36) and ‘Not tired’ (107). The analysis of the structure validity found four dimensions: ‘General Fatigue’, ‘Mental Fatigue’, ‘Reduced Activities’ and ‘Motivation’. The convergent validity showed highly significant correlation (P < 0.001) with a visual analogue scale. The French MFI has been able to distinguish patients with different fatigue levels. Cronbach's alpha measurement of the scale and the subscales are good, cronbach alpha > 0.70. The reproducibility and sensitivity to change in patients who were followed up one month later show satisfactory results. This validation study of the French version of MFI shows that this instrument is valid for clinical application and allows different dimensions of fatigue to be assessed which is of particular interest for physicians, especially for cancer carers, where fatigue assessment is an important aspect of the follow up.