Current evidence supports the use of reactive therapeutic drug monitoring(TDM) to guide treatment changes in inflammatory bowel disease(IBD) patients treated with anti-TNF agents. However, there is less data available on the utility of drug monitoring with vedolizumab(VDZ). To determine if trough levels of VDZ have an impact on treatment based on clinical, laboratory, and endoscopic findings. A total of 64 VDZ trough levels were collected from 49 patients seen at our IBD center from May 2017 to June 2018. Levels were checked either reactively due to ongoing clinical symptoms, endoscopic, biologic markers(fecal calprotectin, ESR, CRP), or radiologic studies suggestive of active disease; or drawn proactively to guide de-escalation of disease therapy. Demographic data was collected including gender, IBD subtype, medications, age, disease duration, BMI, and ethnicity. Inflammatory markers(ESR, CRP), vitamin D, clinical scoring(HBI, UCAI, Mayo), endoscopic and histologic data were obtained within 6 months prior to level being drawn, and at 6 months follow up. Low drug levels were defined as VDZ trough level less than 20 ug/mL. Of the 56 trough levels drawn reactively, 59% were low; and of 8 drawn proactively, 38% were low. There were 32 patients with a total of 36 low VDZ levels. These patients were more likely to be Caucasian(72%, p=0.02), non-obese(BMI≦30) (86%, p=0.001), and less likely to be on combination biologic therapy(20%, p=0.002), with a trend towards lower concomitant immune modulator use(33%, p=0.06). There was no significant difference in gender, male 36% vs female 64%, IBD subtype, CD 44% vs UC 56%, or steroid use 58% vs 42%. Compared to patients with higher drug levels, the lower drug level group had higher inflammatory markers, ESR 20 vs 11.5, p=0.03 and CRP 9.3 vs 4.3, p=0.03, with no difference in the mean vitamin D level or clinical scores. With lower drug levels patients were less likely to be in endoscopic remission 11% vs 35%, p=0.02, or combined endoscopic/histologic remission 8% vs 27% p=0.05. Of the 32 patients with low levels, 10 had follow up trough level drawn after an increase in dosing frequency. All had improvement in their drug levels, but 40% remained under 20 ug/mL. Of the patients who had levels checked reactively, 10 had endoscopic follow up data, of whom 70% had low levels. Fifty percent had endoscopic improvement or remained in remission/mild disease at follow up. The majority of patients who had drug levels checked reactively had levels under 20 ug/mL. In addition, levels checked proactively were frequently low. Low drug levels of vedolizumab were associated with higher inflammatory markers and endoscopic disease activity. This suggests that there may be utility to TDM in order to optimize drug levels and improve outcomes in patients on vedolizumab therapy.