Tumor lysis syndrome (TLS) is exceedingly rare in multiple myeloma because of the relatively slow proliferation and response of the malignant cells. Bortezomib is a novel agent that inhibits proteasome and has shown activity against multiple myeloma. We report 8 episodes of TLS seen in 7 patients with bortezomib therapy, with or without dexamethasone, among 496 patients treated on 3 phase II multicenter studies. Biochemical abnormalities resolved with supportive therapy in 6 patients (including hemodialysis in 2) but proved fatal in 1. Clinicians should be alert for TLS in patients with myeloma with significant disease burden treated with bortezomib because of the potential for rapid onset of cell lysis with this agent.