One prominent feature of patients with the autosomal recessive disease ataxia telangiectasia (AT) is somatic growth retardation. Due to their essential roles in development we examined levels of insulin-like growth factor-I (IGF-I) as well as its main binding protein (IGFBP-3) in a group of AT patients. Growth status of 19 patients was assessed by body mass index (BMI) and nutritional protocols. As suspected, BMI was low in AT patients despite adequate nutrition. Serum levels of IGF-I were found to be below the 3rd percentile in 9 (56%) out of 16 patientsand of IGFBP-3 in 13 (81%) out of 16 patients. Our observations demonstrate that IGF-I and IGFBP-3 levels reflect the impaired growthstatus in patients with AT.