Objective: To investigate if there are any disease-related or methylphenidate-induced aberrations in growth parameters, growth hormone insulin-like growth factor (IGF)- I, IGFBP-3 axis and the thyroid function tests in children with attention deficit hyperactivity disorder (ADHD). Methods: Newly diagnosed and untreated prepubertal children with ADHD were longitudinally followed before and approximately every 4 months after methylphenidate treatment for up to 16 months. Height SDS, weight SDS, BMI SDS, serum GH, IGF-I, IGFBP-3, T-4, free T-4, T-3, and TSH were measured at each visit. Results: All of the examined parameters were within normal limits for age before treatment. Methylphenidate treatment did not significantly affect SDS of height, weight, BMI, IGF-I and IGFBP-3 in the long run. Serum T-4 and free T-4 levels showed modest reductions within normal limits in a time-dependent manner. Conclusions: Prepubertal children with ADHD had normal height, weight, BMI, serum IGF-I and IGFBP-3 and thyroid functions. Methylphenidate treatment had no sustained effects on growth parameters, IGF-I and IGFBP-3 during the follow-up period of this study. However, it caused a mild decrease in total and free T-4 which may warrant further monitoring. Copyright (C) 2005 S. Karger AG, Basel.