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Back ground: The calcineurin inhibitor (CNI) tacrolimus (TAC) has been reported to be effective for induction and maintenance of remission in patients with refractory ulcerative colitis (UC). However, CNI has nephrotoxic potential leading to acute and chronic renal damage in some cases. To date, little is known about the influence of long term administration of oral TAC on renal function in patients with UC. Aim: The aim of our study was to evaluate the incidence and the severity of renal function impairment in UC patient who received TAC treatment. Methods: In this retrospective study, the medical charts of 71 adult patients with steroid-refractory UC treated with TAC between 2012 and 2014 in a single Japanese center were analyzed. In principle, TAC was orally administrated as a 2 week-induction (target trough levels 10-15ng/ml) followed by a maintenance therapy (target trough levels 5-10ng/ml). Estimated glomerular filtration rate (eGFR) was evaluated during the treatment. Acute kidney injury (AKI) was defined by the RIFLE (Risk of renal dysfunction, Injury to the kidney, Failure of function, Loss of function and End-stage kidney disease) consensus criteria using the maximal change in serum creatinine (Scr) and eGFR during the TAC treatment compared with baseline value before treatment. Results: The mean duration of TAC administration was 210 days. At 12weeks, TAC produced a clinical response in 54 patients (76.1%) and remission was achieved in 29 of those 54 (40.8%). The AKI rate during TAC treatment was 46.5% (33 of 71 patients). RIFLE class R (Scr increase > 1.5 times or eGFR decrease > 25%) accounted for 27 patients (38.0%), and RIFLE class I (Scr increase > 2 times or eGFR decrease > 50%) for six (8.5%). The AKI rate was 76.8% (10/13) in patients who had been administrated TAC for more than 1 year and 37.5% (18/48) in patients with TAC treatment within 6 months (p=0.006). After withdrawal of TAC, renal function impairment (eGFR decrease > 25%) was still observed in 10 patients (14.1%). Conclusions: Oral TAC therapy appears to be effective for patients with refractory UC. However, renal function impairment was frequently observed during this treatment. Thus, careful monitoring of renal function must be required to avoid irreversible chronic renal damage during long-term administration of TAC.