Objectives: to determine the prevalence and key factors associated with fatigue in chronic low back pain patients. Methods: cross-sectional study of 215 chronic low back pain patients from three health care centers and two industrial corporations. The crude prevalence of fatigue and its 95% confidence interval (CI) were calculated. Associations between fatigue and the independent variables were measured. Results: the prevalence of fatigue among the participants was 26.0% [95% CI: 20.3 - 32.5]. Fatigue was independently associated with depression and self-efficacy. An increase of one unit in the score of depression increased the risk of fatigue by 9%; an increase of one unit in the score of self-efficacy reduced the risk of fatigue by 2%. Conclusions: fatigue was prevalent in chronic low back pain patients and associated with depression and self-efficacy. 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