BACKGROUND: Familial Mediterranean fever (FMF) is an inherited disorder caused by a number of mutations of the Mediterranean fever (MEFV) gene, coding a protein named pyrin that acts as a major regulatory component of the inflammasome. The first-line drug for FMF treatment is colchicine, but 10% of patients with FMF do not respond well to colchicine. Although the efficacy of tocilizumab (TCZ), which is a recombinant, humanized, antihuman interleukin 6 (IL-6) receptor monoclonal antibody, has been reported to prevent FMF attacks, the effects of TCZ on individuals with colchicine-resistant or colchicine-intolerant FMF have not been evaluated in a randomized clinical trial. METHODS/DESIGN: In this phase III, investigator-initiated, multicenter, double-blind, randomized, parallel-group trial, the efficacy and safety of TCZ will be compared with placebo in patients with colchicine-resistant or colchicine-intolerant FMF. The study will be conducted in nine centers in Japan. Participants (n = 24) will be randomly assigned to receive 162 mg of TCZ (n = 12) or placebo (n = 12) administered subcutaneously once weekly for 24 weeks. Rescue treatment will be allowed if rescue criteria are met. A primary endpoint is the number of fever attacks until 24 weeks. Secondary endpoints include the number of occurrences of accompanying symptoms during attacks; the time until a fever attack occurs; the duration of fever attacks; serum C-reactive protein and serum amyloid A; 36-item Short Form Health Survey; general evaluation by a physician (100-mm visual analogue scale); body temperature; the percentage of subjects who achieve FMF 50 at 12 weeks and 24 weeks; and pharmacodynamic assessment, including the measurement of serum TCZ level and soluble IL-6 receptor. DISCUSSION: The study is expected to produce evidence regarding the efficacy of a potential new therapeutic agent, TCZ, in improving the clinical course and outcome for patients with colchicine-resistant or colchicine-intolerant FMF. TRIAL REGISTRATION: University Hospital Medical Information Network Clinical Trials Registry, UMIN000028010 . Registered on 7 July 2017.