Studies of measurement of maximal aerobic uptake in patients with chronic low back pain have shown inconsistent results and none has focused on clinical endpoints of ergometry tests.To determine the level of cardiorespiratory fitness and to establish factors limiting the maximal effort during the ergometry.Patients with chronic low back pain performed a graded maximal exercise test. Clinical endpoints of the test were determined and grouped as "maximal effort" and "symptom-limited effort" endpoints.101 patients aged 29.8 +/- 7.5 achieved a VO2 max value of 30.0 (+/- 7.27) ml.kg(-1).min(-1). In men and women independently, a linear regression analysis demonstrated that VO2 max was inversely and weakly related to age. When compared to normative categories of VO2 max, men and women were situated in the fair category. Quadriceps/leg fatigue was reported by 47.5% of patients and was the most frequent limiting factor of the tests. "Symptom limited effort" endpoints were reported by 54.4% of the subjects.Clinical limiting factors of maximal exertion interfere the achievement of maximal oxygen uptake in patients with low back pain during ergometry. Further, there exists an association among the clinical endpoints of the tests and the VO2 value achieved. CLBP patients have a lower level of aerobic fitness than healthy controls.