Nilotinib, a second-generation tyrosine kinase inhibitor with 20- to 30-fold greater potency than imatinib, was developed to overcome imatinib intolerance or resistance. Recently, nilotinib has been approved as a first-line treatment for chronic myelogenous leukemia in the US and Japan. Tumor lysis syndrome (TLS) is an extremely rare adverse event that can occur during treatment with nilotinib, with only a few reported cases to date. Herein, we report two patients who developed TLS soon after the start of treatment with nilotinib. While in the first case, which co-presented with underlying mild-to-moderate renal insufficiency due to polycystic kidney disease, the TLS resolved on discontinuation of the drug, the second patient, who had an exceedingly high white blood cell count, presented with disseminated intravascular coagulation and severe liver injury triggered by TLS that developed after the start of nilotinib treatment, and died of multiple organ failure. Therefore, caution is necessary when this drug is used in the first-line setting in patients with renal insufficiency or a high tumor burden.