Objective. The purposes of this study were to measure the movements of the lumbar spine produced by rotational mobilisation, and to study the effects of different grades of mobilisation on the movements produced. Design. Kinematics of rotational mobilisation was assessed with an electromagnetic tracking device. Background. Rotational mobilisation is frequently used in the treatment of back pain, but there was no information on its mechanical effects. Methods. Movements of the lumbar spine were measured in 14 healthy volunteers when they were subjected to grades I to IV left rotational mobilisation. Results. In the starting positions, the spines were found to be flexed, axially rotated to the left and laterally bent to the right. As the mobilisation grade increased, the spine was axially rotated further into the range. Rotational mobilisation was found to produce oscillatory movements of the lumbar spine in all three anatomical planes. It produced axial rotation which was accompanied by lateral bending in the opposite direction and sagittal rotation. The mean frequency of the oscillatory movements was 1.4 Hz. The amplitude of the oscillations was small, and was found to be increased in grades II and III mobilisation. Conclusion. Rotational mobilisation may be able to restore lost movements of the lumbar spine in any of the three anatomical planes. Relevance. An understanding of the kinematics of mobilisation will allow therapists to gain insight into its mechanical and perhaps therapeutic effects.