Ximoprofen is a new propionic NSAID which has previously demonstrated its efficacy at a daily dose of 30 mg. The aim of the study was to evaluate the efficacy of different daily dosages of Ximoprofen in patients with active ankylosing spondylitis. For 2 weeks 5 parallel groups were studied: placebo and Ximoprofen at 5, 10, 20 and 30 mg daily. Response to treatment was defined as an improvement in pain (VAS 100 mm) of at least 50% during the study. 285 out of the 332 screened patients were included. At the end of the study, the percentage of responders was higher in the Ximoprofen groups (54%, 41%, 53%, 56% in the 5, 10, 20 and 30 mg groups, respectively) than in the placebo group (21%). The clearest dose related effect of Ximoprofen observed occurred after one week of treatment. This study 1/confirms the efficacy of Ximoprofen at a 30 mg daily dosage, 2/shows the persistence of this efficacy at lower dosages, 3/suggests that ankylosing spondylitis is a sensitive and relevant human model to assess NSAIDs at an early stage of clinical evaluation.