Obesity has become a pandemic, affecting both children and adults. We sought to determine the effect of obesity among 200 children who were prospectively enrolled on a multicenter cord blood transplant (CBT) trial. All patients received myeloablative preparative regimens. Children were classified into groups according to body mass index percentile. Normal weight was defined as body mass index between the 5th and 85th percentile (n=117), overweight between the 85th and 95th percentile (n=35) and obesity above 95th percentile (n=39) for age and gender. A total of 55 patients (27%) had AML, 113 patients (57%) had ALL and 32 patients (16%) had other malignant diseases. There was no evidence for a difference in all major characteristics among the groups. Time to neutrophil and platelet engraftment, TRM, risk of acute GVHD, disease-free survival and OS were not significantly different in overweight or obese patients compared with normal weight patients. There was a trend towards increased risk of chronic GVHD in obese patients (P=0.05) compared with normal weight patients. In conclusion, there is insufficient evidence from this sample that obesity has an effect on multiple outcomes after unrelated CBT in children with malignant diseases.