INTRODUCTION Ofatumumab, a fully-human anti-CD20 monoclonal antibody, is indicated for the treatment of relapsing forms of multiple sclerosis (RMS). However, the potential impact of ofatumumab on microglial activation in MS is unknown. The goal of this study is to determine the effect of ofatumumab on microglial activation using [F-18]PBR06 positron emission tomography (PET) targeting 18kilodalton-translocator protein (TSPO) in RMS patients. METHODS 20 [F-18]PBR06 PET scans in 5 RMS patients (mean age 40.2±12 years, 4 females, median expanded disability status scale 3.0, mean timed-25 foot walk 4.25±0.54 seconds) were performed prior to, and at days 5, 28 and 90 after initiating ofatumumab. Peripheral CD19 counts and clinical evaluations were also performed. Individualized z-score maps of brain parenchymal microglial activation were generated by voxel-by-voxel comparison between each subject’s PET SUVR images and a control dataset of 9 healthy individuals. Glial activity load on PET (GALP) was calculated as the sum of voxel-by-voxel z-scores ?4 in the lesional and perilesional normal-appearing white matter (LWM and P-NAWM), cortical grey matter (CoGM) and thalamic (Th) regions of interest (ROI) in standard atlas space. Mean ROI GALP scores, peripheral CD19 counts and clinical measurements over 90 days were compared with baseline values. p<0.05 was considered statistically significant. RESULTS Mean CoGM-GALP was significantly decreased at 90 days but not at days 5 or 28, after initiation of ofatumumab, as compared to BL (0.75±0.09 vs. 0.93±0.06, -24%, p0.05). CONCLUSIONS Ofatumumab treatment was associated with decreased cortical grey matter microglial activation at 3 months and was preceded by peripheral CD19+ cell depletion at day 5, which may suggest an indirect, downstream effect of B-cell depletion on microglial activity in RMS patients. Further studies on effects of ofatumumab on microglial activation are warranted.