Siltuximab, a chimeric monoclonal antibody with high affinity and specificity for interleukin-6, has been shown to enhance anti-multiple myeloma activity of bortezomib and corticosteroid in vitro. We evaluated the safety, pharmacokinetics, immunogenicity, and antitumor effect of siltuximab in combination with bortezomib and dexamethasone in Japanese patients with relapsed or refractory multiple myeloma. This open-label, phase 1, dose-escalating study used two doses of siltuximab: 5.5 and 11.0 mg/kg (administered on day 1 of each 21-day cycle). In total, nine patients were treated. The most common grade 3/4 adverse events, lymphopenia (89 %) and thrombocytopenia (44 %), occurred in patients receiving both doses of siltuximab; however, no dose-limiting toxicities (DLTs) were observed. Following intravenous administration of siltuximab at 5.5 and 11.0 mg/kg, the maximum serum concentration and the area under the curve from 0 to 21 days and from 0 to infinity increased in an approximately dose-proportional manner. Mean half-life, total systemic clearance, and volume of distribution were similar at doses of 5.5 and 11.0 mg/kg. Across both doses, six of the nine patients had complete or partial response (22 and 44 %, respectively). In conclusion, as no DLT was observed, the recommended dose for this combination is 11.0 mg/kg once every 3 weeks. The study is registered at http://www.clinicaltrials.gov as NCT01309412.