BACKGROUND: There is still no definite treatment for refractory Kawasaki disease (KD). In this pilot study, we evaluated the safety and efficacy of a new protocol consisting of sivelestat sodium hydrate (SSH) combined with additional i.v. immunoglobulin (IVIG) for KD resistant to initial IVIG therapy. METHODS: This study is a prospective non-randomized, open-label and single-arm study undertaken in a population of refractory KD patients at Chiba University Hospital from December 2006 to March 2016. The subjects had KD resistant to initial IVIG (2 g/kg) and received SSH (0.2 mg/kg/h for 5 days) combined with additional IVIG (2 g/kg) as a second-line therapy. We evaluated the safety and efficacy of the treatment during the study period. RESULTS: Forty-six KD patients were enrolled in this study and no serious adverse event was noted. Of these, 45 patients were evaluated for the incidence of coronary artery lesions, which occurred in one patient (2.2%; 95% CI: 0.5-15.2). Twenty-eight (62.2%) responded promptly and were afebrile after the therapy. The median total duration of fever was 8 days (range, 6-28 days). CONCLUSIONS: Additional IVIG combined with SSH as a second-line therapy for KD refractory to initial IVIG therapy was safe and well tolerated and could be a promising option for severe KD. Further investigations are expected to clarify the safety and timing of SSH treatment for KD.