To establish the level of cardiorespiratory fitness and the rate of decrease in maximal aerobic capacity according to age in patients with chronic low back pain and compare these with normative data.Prospective case series with historical controls.Seventy patients with chronic low back pain.A maximal cycle ergometer protocol was used to measure VO2max, heart rate, respiratory exchange ratio and blood lactate levels.Seventy patients achieved absolute and normalized for weight VO2max values of 2.17 (standard deviation (SD) 0.65) l/min and 30.79 (SD 7.77) ml/kg/min, respectively. Absolute VO2max was poorly related to age in both men and women with chronic low back pain (r = -0.22 and r = -0.28, respectively). VO2max normalized for weight was also inversely related to age in both men and women (r = -0.36 and r = -0.42, respectively). The rate of VO2max decline between 20 and 59 years was -3.3 ml/kg/min/decade for the entire population and -1.2 and -5.4 ml/kg/min/decade in men and women, respectively.The level of physical fitness of patients with chronic low back pain is comparable to the physical fitness of healthy, but poorly conditioned subjects. Patients with chronic low back pain show a VO2max decline with ageing that is slower than of active subjects.