INTRODUCTION: Between 20% and 40% of patients with severe ulcerative colitis (UC) are either steroid-refractory UC (SRUC) or steroid-dependent UC (SDUC). Tacrolimus is an oral and relatively inexpensive drug, which has been extensively used in Japan for steroid-refractory and steroid-dependent disease. METHODS: Patients diagnosed with SDUC/SRUC were treated with tacrolimus 0.05-0.1 mg/kg in this prospective study. Clinical Mayo score (CMS) and UC Endoscopic Index of Severity (UCEIS) were evaluated prior to starting the drug and subsequently after 8 weeks. 5-Aminosalicylic acid agents (5-ASA) and immunomodulators were continued if the patients were previously on these drugs. Clinical response at 8 weeks was defined as decrease in CMS by at least 3 points. Clinical remission was defined as CMS </=2 and combined remission as CMS</=2 with UCEIS <3. RESULTS: Fifty-two patients (29 males) with a mean age of 35.1+/- 12.8 years with predominantly E3 disease (71%) were prospectively evaluated in this study. SDUC and SRUC were diagnosed in 31 and 21 patients, respectively. Seven failed treatment within 8 weeks, four were subjected to surgery, and 3 were switched to infliximab. Forty-two patients continued tacrolimus for 8 weeks. Mean CMS and UCEIS prior to starting tacrolimus were 6 +/- 1.1 and 4.8 +/- 1.1, respectively. At 8 weeks, median CMS and UCEIS decreased to 2.6 +/- 1.7 and 2.7 +/- 1.3, respectively. Clinical response was documented in 29 patients (56%) at week 8 whereas clinical remission was seen in 25 patients (48%). Combined clinical and endoscopic remissions were seen in 18 patients (35%). Except for a single patient who developed reversible renal dysfunction, no other adverse event was observed. CONCLUSION: Our results show that tacrolimus is effective in inducing a clinical response in 56% of patients with SDUC and SRUC. In view of its low cost and safety profile, it may be considered first-line therapy for SDUC/SRUC.