In this study we will try to demonstrate that infliximab administered at 5mg/kg to patients with juvenile onset SpA over a period of 12 weeks will have more efficacy than placebo and that it will be well tolerated. At the end of this phase, patients will go into a 52-week open extension to demonstrate sustained efficacy, safety, and tolerability of infliximab We will include 34 patients with juvenile onset SpA unresponsive to standard treatment. Efficacy will be assessed by counting the number of actively inflamed joints and a number of other parameters.