CPT-11/5-FU/LV is an active combination agent in the treatment of colorectal cancer, but one of its dose-limiting toxicities is diarrhea. PHY906 has been shown to reduce the severity of CPT-11–induced toxicity without compromising antitumor efficacy in in vivo animal models. Additionally, there has been a long historical experience in the Far East demonstrating safety of PHY906 in humans. Thus, we are conducting this double-blind, placebo-controlled study to evaluate the safety and tolerability of PHY906. This dose escalation study will also examine the effect of PHY906 (1.2, 2.4, and 3.6 g/day) on the pharmacokinetics of CPT-11 and 5-FU after concomitant administration with CPT-11, 5-FU, and LV, and on the severity of CPT-11–induced toxicities such as diarrhea.