The Polhemus Navigation Sciences 3Space Isotrak system was used to measure the range of lumbar spinal motion of 57 patients with ankylosing spondylitis. Forty-three of these attended voluntary exercise sessions for an average of one and a half hours per week while 14 did not participate in any formal exercise groups. Exercising patients fell into two groups: those attending moderate and those attending vigorous exercise sessions. Results for exercising patients obtained immediately pre- and post- a single exercise session showed a small but significant increase in extension for the vigorous exercise group but no significant changes in any other movement for either of the groups. In a group of 44 patients (33 exercising, 11 non-exercising) who were followed-up over a 2 to 6 month period, slight loss of flexion (5.5°) and lateral bend (3°) was observed but there was no change in range of extension.