BACKGROUND: Inflammatory bowel diseases, Crohn's disease (CD) and ulcerative colitis (UC), are associated with reduced quality of life (QoL). By using questionnaire tools called patient-reported outcome measures (PROM), patients' well-being and health-status can be measured. The aim of this study was to identify subgroups at risk of being missed in questionnaire monitoring and assess QoL and variability of responses over time. METHODS: CD or UC, age >/=18 years, receiving biological treatment subcutaneously or intravenously, 01 August 2018 to 31 January 2020, at Karolinska University Hospital, were included. Patients completed standardised and validated questionnaires for QoL-measurements; Short Health Scale (SHS) and EuroQol 5-dimension-index (EQ5D). RESULTS: 412 patients, 287 (70%) Crohn's disease, 125 (30%) ulcerative colitis, 267 (65%) males, median age: 33 (range 18-85). Patients receiving subcutaneous treatment completed PROM questionnaires significantly less frequently compared with intravenous treatment (multiplicative factor 6.5, 5.7-7.5 95% CI). Reduced QoL was seen for intravenous treatment (multiplicative factor 2.0, 0.5-3.5 95% CI) and active disease (multiplicative factor -4.0, -6.1 to -1.9 95% CI). Greater variability in responses was seen in active disease, anaemia, faecal calprotectin >/= 250 mg/kg. CONCLUSIONS: Patients receiving subcutaneous treatment, equivalent to home-based treatment, completed significantly fewer PROM questionnaires and are therefore less monitored. It is therefore important to offer different modes of questionnaire administration when monitoring a heterogeneous patient population especially as we see a shift towards oral forms of therapy.