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. Knowing these factors can direct strategies for prevention and control of fatigue in chronic low back pain patients.