Background/purposePreclinical vascular inflammation models have demonstrated effective suppression of arterial wall lesional T cells through inhibition of Janus kinase 3 and JAK1. However, JAK inhibition in patients with giant cell arteritis (GCA) has not been prospectively investigated.MethodsWe performed a prospective, open-label, pilot study of baricitinib (4 mg/day) with a tiered glucocorticoid (GC) entry and accelerated taper in patients with relapsing GCA.Results15 patients were enrolled (11, 73% female) with a mean age at entry of 72.4 (SD 7.2) years, median duration of GCA of 9 (IQR 7–21) months and median of 1 (1–2) prior relapse. Four (27%) patients entered the study on prednisone 30 mg/day, 6 (40%) at 20 mg/day and 5 (33%) at 10 mg/day. Fourteen patients completed 52 weeks of baricitinib. At week 52, 14/15 (93%) patients had ≥1 adverse event (AE) with the most frequent events, including infection not requiring antibiotics (n=8), infection requiring antibiotics (n=5), nausea (n=6), leg swelling (n=2), fatigue (n=2) and diarrhoea (n=1). One subject required baricitinib discontinuation due to AE. One serious adverse event was recorded. Only 1 of 14 (7%) patients relapsed during the study. The remaining 13 patients achieved steroid discontinuation and remained in disease remission during the 52-week study duration.ConclusionIn this proof-of-concept study, baricitinib at 4 mg/day was well tolerated and discontinuation of GC was allowed in most patients with relapsing GCA. Larger randomised clinical trials are needed to determine the utility of JAK inhibition in GCA.Trial registration numberNCT03026504.