OBJECTIVE: To assess the clinical effect and acute inflammatory and chronic bony changes of the sacroiliac (SI) joints as detected by magnetic resonance (MR) and computerized tomography (CT) imaging in patients with ankylosing spondylitis (AS) during treatment with etanercept. METHODS: The all 16 patients with AS were treated with etanercept 25mg twice weekly subcutaneously and the clinical response was assessed by standardized parameters. Active inflammatory lesions and chronic bony changes of SI joints were assessed by the MR and CT images of the sacroiliac joints before and after treatment with etanercept. RESULTS: The mean disease duration was 13.1+/-0.69 years and the mean duration of treatment was 14.9+/-4.86 weeks. The mean BASDAI and BASFI score decreased significantly after etanercept treatment. The regression of active inflammation of sacroiliac joint was seen only in a patient with early disease stage of AS (18 months). However, no significant changes in acute inflammatory and chronic bony changes of the SI joints were found on MR and CT images after treatment of etanercept. CONCLUSION: Etanercept treatment showed good clinical response. However, no decrease in acute inflammatory and chronic bony changes of the sacroiliac joints was shown on MR and CT images in the AS patients had long-standing disease. Thus, it is suggested that etanercept should be used in early disease stage to obtain the radiographic improvement of sacroiliac joints.