Thirty-four patients who met ESSG criteria for spondylarthropathy and were eligible for anti-TNFalpha treatment (infliximab) were enrolled in this open-label study lasting 14 weeks. The aims were to evaluate the progression of sacroiliitis by means of MRI, and to determine the positive predictive value of this exam for the treatment response. Patients underwent MRI of the sacroiliac region at baseline (W0), and also at 14 weeks if the baseline MRI showed sacroiliitis. Two blinded readers reviewed all imaging studies. The patients also had a physical examination and ESR and CRP assays at W0 and W14. Sacroiliitis was found in 22 patients (65%) at W0, but only 18 of these patients had a second MRI at W14, for technical reasons. After 14 weeks of therapy, MRI signs of sacroiliac inflammation diminished by 77.7% on average. Clinical and biological parameters also improved. However, MRI was not predictive of the treatment response. Sacroiliac MRI seems to be interesting for objective therapeutic evaluation and monitoring of patients with spondyloarthropathy.