Objectives: To examine the effect of accelerated infliximab induction in children with moderate to severe UC. Design: A multi-center, prospective, randomized, open label study. Setting: Pediatric gastroenterology centers. Participants: Children 6 year to 17 years (Overall, 84 patients) with moderate to severe UC who are corticosteroid dependent/resistant thus planned to receive infliximab induction. Intervention: Group 1 (intervention) will receive an accelerated induction at 0,1,3 weeks (5 mg/kg) and then at week 7,11,15. Group 2 (standard) will receive a per protocol induction at 0,2,6 weeks (5 mg/kg) and then at week 14. Drug levels will be obtained prior to each infusion in each group (up to week 20). Further maintenance will be planned according to drug levels at weeks 15 and 14, respectively. Follow-up will continue without further interventions till 52 weeks following induction. Main outcome measure: Clinical remission, on infliximab at week 20. Secondary outcome measures: 1. Colectomy free rates at week 20 and 52. 2. Clinical remission on infliximab at week 52. 3. Drug levels and anti-drug antibodies prior to last study infusion. 4. Anthropometric and laboratory measures including calprotectin at the end of induction, week 20 and week 52 5. Changes in fecal microbiome, virome and bile acids content. Sample size: In order to demonstrate 30% difference in clinical remission rate between groups is significant, we will need to study 36 children in each group to be able to reject the null hypothesis that the failure rates between the groups are equal with probability (power) of 80% and a type I error probability of 0.05.