Introduction: Ulcerative colitis (UC) includes a dysregulated immune response. The conventional therapy includes immunosuppressants, and biologics targeting inflammatory mediators, but these are often inadequate, or subjects become unable to tolerate them. Areas covered: QUASAR: the induction and maintenance components of the phase 3 trial of guselkumab, which inhibits IL-23 by dual mechanisms, in subjects with moderate-to-severe UC. QUASAR enrolled those that had an inadequate response and/or intolerance to corticosteroids, immunosuppressants, biologics, or Janus kinase (JAK) inhibitors. In both parts of the trial, guselkumab improved clinical remission with no excess of adverse events. Expert opinion: For those enrolled throughout, after the maintenance part, the benefit with guselkumab on clinical remission was 24% percentage points (45 vs 21%), which is relatively small. There is no direct comparison of guselkumab with other IL-23 inhibitors in UC. Indirectly comparing trials suggests the clinical remission rates at the end of the trials was higher with guselkumab than with the other approved IL-23, inhibitors, mirikizumab or risankizumab (17 or 15% points, respectively). Thus, guselkumab may be more efficacious than the other 1 L-23 antagonists, possibly due to its additional action to block the CD64 receptor. However, this needs to be tested in a direct comparison trial.