Forty consecutive patients will be recruited from the rheumatology clinic of the Prince of Wales Hospital with AS meeting the modified New York criteria with active disease as defined (see below). They will be randomized to receive MTX 7.5 mg/week initially with a weekly 2.5mg increment until 15mg/week dosage is reached,( i.e by week 6) or a placebo together with folic acid 5mg daily for a period of 16 week. After 16 weeks, all patients will receive infliximab at 5 mg/kg per dose, at weeks 16, 18, and 22 (3 doses), and will continue with MTX 15 mg/week or placebo. Thereafter, they will be followed up at week 30, 38 weeks. MRI changes in the sacroiliac joints (SI) before and after infliximab treatment.