Background: Risankizumab, an anti-interleukin-23 antibody, was superior to placebo in achieving clinical and endoscopic remission at week 12 in a randomised, phase 2 induction study in patients with moderately to severely active Crohn’s disease. The efficacy and safety of extended intravenous induction and/or subcutaneous maintenance therapy with risankizumab was assessed. Methods: Following 12-week, double-blind, randomised, induction treatment comparing 200 mg or 600 mg intravenous risankizumab to placebo every 4 weeks, patients without deep remission, defined as clinical (Crohn’s Disease Activity Index <150) and endoscopic remission (Crohn’s Disease Endoscopic Index of Severity [CDEIS] ≤4 [≤2 for patients with isolated ileitis]), received open-label 600 mg intravenous risankizumab (every 4 weeks) and patients in deep remission underwent washout until week 26 (Period 2). At week 26, patients in clinical remission received maintenance treatment (Period 3) with 180 mg subcutaneous risankizumab (every 8 weeks). Efficacy endpoints included clinical and endoscopic response and remission at weeks 26 (Period 2) and 52 (Period 3) respectively; safety was assessed through both periods. Study registration: ClinicalTrials.gov, NCT02031276. Findings: In Period 2, 101 patients were treated with 600 mg risankizumab resulting in an increase in clinical remission rates at week 26 versus week 12 for all original designated treatment groups: 55% versus 18%, 59% versus 21%, and 47% versus 26% for placebo, 200, and 600 mg risankizumab, respectively. Of the 62 patients receiving maintenance treatment, 54 completed treatment. At week 52, clinical remission was maintained by 71% of patients; endoscopic remission and response (>50% CDEIS reduction from baseline) was achieved by 35% and 55% of patients, respectively, and 29% of patients achieved deep remission. Risankizumab was well tolerated with no new safety signals. Interpretation: Extended induction treatment with open-label intravenous risankizumab was effective in increasing clinical response and remission rates at week 26. Open-label subcutaneous risankizumab maintained remission till week 52 in most patients who were in clinical remission at week 26. Selective blockade of interleukin-23 warrants further evaluation as treatment for Crohn’s disease.