The aim of this investigation was to assess the reproducibility of the isokinetic trunk muscle performance measurement among patients with different degrees of low-back pain. Twenty-two healthy volunteers, 20 patients with mild and 18 patients with severe low-back pain participated in isokinetic measurements. Lidoback isokinetic dynamometer was used. The measurements were performed with the subjects standing, using velocities of 60, 90 and 120 degrees/second. Five repetitions were performed at each velocity. All subjects were tested three times with a 1-week interval between the tests. Peak torque, average peak torque, coefficient of variation, total work done and peak torque to body weight ratio were calculated for each velocity for both flexion and extension. The results showed that in every measurement peak torque, average peak torque, peak torque to body weight ratio and total work done correlated with each other very strongly both in flexion and extension (r > or = 0.9). The average peak torques increased in further measurements. The change had a strong correlation with the severity of the back problem, which was evaluated by means of the Oswestry disability index. The critical value was found to be 20% in the Oswestry index: values above this meant big changes between measurements and values below this meant small changes between measurements.