Thirty-four patients allocated and randomized to infliximab 5 mg/kg or placebo. Randomization: restricted by blocks of four and by stratification in two diagnostic categories (undifferentiated SpA and ankylosing spondylitis). Efficacy analysis: change of the primary efficacy measure and secondary measures on weeks 2, 6, and 12, and any visit of discontinuation compared to week 0 and compared to changes in the placebo group.