This paper describes a new method of measuring the three-dimensional movements of the lumbar spine in real time. The measurement system consisted of solid-state gyroscopes which were attached to the trunk. They measured the angular rates of rotations in three dimensions, which were then integrated to obtain the orientation. The sensors contained gravitometers and magnetometers which provided additional information for eliminating any drift of the gyroscopes. The reliability of the data provided by the gyroscopic system was examined in a group of 19 young healthy subjects. The similarity of the movement-time curves obtained in three repeated measurements was assessed by the coefficient of multiple correlation. The coefficients were found to be high, ranging from 0.972 to 0.991. The reliability of the data was slightly lower for measuring axial rotation. The device did not only quantify the kinematic patterns in the primary plane of movements, but also the accompanying movements in the other planes. Flexion and extension was found to be mainly confined to the sagittal plane, whereas lateral bending and axial rotation always accompanied each other. It was concluded that the inertial tracking device would be a useful tool for clinical measurement as well as biomechanical investigations.