Impairment of glucose metabolism (in particular insulin resistance and type 2 diabetes mellitus) has been reported in patients who have undergone hematopoietic SCT (HSCT) during childhood, especially those treated with TBI. This pilot study was conducted to determine prevalence of and possible underlying mechanisms for impaired glucose homeostasis in young adults treated with HSCT and TBI and who were not previously known to have diabetes mellitus. A total of 10 subjects (6 males, 4 females) were evaluated. Mean ages were 13.0+/-1.0 years at the time of TBI and 24.0+/-1.1 years at the time of this study. Five subjects had laboratory evidence of insulin resistance using the homeostasis model assessment and the quantitative insulin sensitivity check index indices. Two of these subjects had impaired fasting glucose and four had decreased plasma insulin-like growth factor 1 levels. All 10 subjects had evidence of abdominal obesity. Insulin resistance is frequently observed in adult survivors of HSCT treated with TBI in childhood. Underlying mechanisms may include radiation-induced growth hormone deficiency and changes in body composition.