We report a prospective Phase II study of efficacy and toxicity for oral treosulfan in advanced Ewing sarcoma. Twenty patients, median age 19 years (range 7–39) from five UK sites, were treated with oral treosulfan 1 g/m2 daily for 7 days in 28. Primary endpoint was objective response rate. Best response was stable disease in one patient. All patients died of progressive disease, after median 6.41 months. Median progression free survival was 1.8 months. Toxicity was minimal. No activity was demonstrated for treosulfan at this dose. Progression free survival data should be able to be used for comparison when planning future clinical trials. Pediatr Blood Cancer 2015;62:158–159. © 2014 Wiley Periodicals, Inc.