Abstract Purpose: To determine the dose-limiting toxicity (DLT) and the maximum tolerated dose of ecteinascidin-743 (ET-743, Yondelis) in children with refractory solid tumors, to establish the recommended dose for pediatric phase II trials, and to characterize the pharmacokinetics of ET-743 in children. Experimental Design: ET-743 was administered as a 3-hour i.v. infusion every 21 days. The starting dose was 1,100 μg/m2 with planned dose escalation of 200 μg/m2 increments. Pharmacokinetic sampling was done during the first treatment course. Results: Twelve evaluable patients received a total of 29 courses. One grade 4 DLT (prolonged grade 4 neutropenia) was noted at the first dose level. At the second dose level (1,300 μg/m2), there were two DLTs (reversible grade 4 elevations of hepatic transaminase); hence the maximum tolerated dose was defined as 1,100 μg/m2. Overall, reversible hepatic toxicity, manifested as grade 3 or 4 elevations in hepatic transaminase, occurred in more than 50% of the patients. No grade 3 or 4 thrombocytopenia was reported at either dose level and only one episode of isolated creatine phosphokinase grade 4 elevation was observed. One complete response was documented after six courses in a patient with metastatic Ewing sarcoma. The pharmacokinetics of ET-743 in 8 children was characterized by a terminal disposition phase with a mean half-life of 43.8 ± 18.4 hours, a total body clearance of 28.2 ± 10.5 L/h/m2, and a 959 ± 807 L/m2 steady-state apparent volume of distribution. Conclusion: ET-743 is safe. The phase II recommended dose of ET-743 administered as a 3-hour i.v. infusion following premedication with dexamethasone is 1,100 μg/m2.