OBJECTIVES: It is suggested that serious levels of fatigue are present in nearly half of patients with OA. However, it is unclear which dimensions of fatigue are involved, if fatigue is related to pain and physical function, and if fatigue is influenced by therapy. The aims of this study were to measure levels of different dimensions of fatigue before and after evidenced-based conservative treatment and to investigate the association between fatigue and pain and physical function in patients with knee or hip OA. METHODS: In this observational cohort study, levels of different dimensions of fatigue were measured in knee and/or hip OA patients before and after 12 weeks of conservative treatment. Cross-sectional and longitudinal relations between (change in) fatigue dimensions and (change in) pain or physical function were studied using association models, controlling for predefined possible confounders. RESULTS: A total of 231 patients was included, with 47% experiencing severe fatigue. A small decrease in levels of fatigue was seen after standardized treatment. The level of fatigue severity was cross-sectionally and longitudinally associated with physical function, whereas the level of physical fatigue was cross-sectionally and longitudinally associated with pain and physical function. No confounders were identified. CONCLUSIONS: Important levels of fatigue are common in knee and hip OA patients. After evidence-based tailored conservative treatment targeted to improve pain and physical function, a small decrease in fatigue levels was found. Reduction in levels of different fatigue dimensions were related to the change in physical function and pain.