Abstract Background Vedolizumab (VDZ) dose optimisation (DO), by interval shortening from 8-weekly (Q8W) to 4-weekly (Q4W) dosing, is used for patients with secondary loss of response. This report presents outcome data on patients receiving DO in real-world clinical practice in Belgium. Methods The Belgian VDZ Registry (ENcePP EUPAS6469) enrolled 202 VDZ-treated ulcerative colitis (UC) or Crohn’s disease (CD) adult patients (26% with no prior use of anti-TNF therapy) from 19 centres. The median length of VDZ therapy prior to enrolment was 11 months. Patients were followed-up every 6 months with the assessment of IBD features, use of biologics, and disease activity. Clinical remission was defined as Harvey–Bradshaw Index (HBI) <5 or partial Mayo Score (pMS) <2, and clinical response as a 2+ point improvement in pMS or a 3+ improvement in HBI. Results During a median follow-up of 19 months from enrolment, 57 (28%) patients (41 CD and 16 UC) received VDZ Q4W due to secondary loss of response. Q4W was mostly used in patients with CD or with prior anti-TNF therapy failure. The median starting point for Q4W dosing was 16 months after the start of VDZ (interquartile range (IQR) 8–27 months) and median duration of Q4W dosing was 4 months (IQR 2–8 months). After changing to Q4W dosing 44% achieved clinical remission, 3% clinical response, and 53% showed no improvement (Table 1). Among the 17 patients with clinical remission/response on Q4W dosing, 53% de-escalated back to Q8W, and continued with Q8W for a median duration of 12 months, 23.5% remained on Q4W with clinical remission, and 23.5% eventually stopped VDZ due to loss of response. A limitation of this study is that it did not systematically collect data on DO prior to recruitment, hence the proportion of patients receiving DO may be higher than reported here. Conclusion These real-world data show DO plays an important role in management of UC and CD. In this study, 28% of patients received DO following the secondary loss of response to Q8W therapy. ...