 In Japan, patients with relapsed or refractory T-ALLT-LBL represent an extremely small patient population. While the small number of patients presents a practical limitation to the size of a clinical trial, patients whose disease has not responded to or has relapsed after treatment with multiple prior chemotherapy regimens have no accepted standard therapies available. Japanese leukemia experts have expressed interest in evaluating 506U78 in Japanese patients with relapsed or refractory T-ALLT-LBL. In order to obtain safety, tolerability, and pharmacokinetic data of 506U78 in Japanese patients, this study is designed to maximize the contribution of each available patient. 