The main feature of congenital hypothyroidism (CH) is a total or partial absence of thyroid hormone, caused by a developmental disorder of the thyroid gland. Introduction of neonatal thyroid screening allows early treatment with replacement hormone. As a result the prognosis of CH has greatly improved but neuropsychological problems do still occur. Claims that early-treated CH is associated with impulsivity were tested with a memory search task comprising 2 response probability conditions: a baseline condition in which "yes" and "no" responses were equally probable and a response bias condition in which the probability of a "no" response was raised to 70%. Impulsivity was defined as the ability to stop and change a response set and was assessed from the amount of inaccurate "no" responses made. Comparisons were made between 47 children with early-treated CH and 35 controls at the age of 7.5 years. Children with early-treated CH were notably slow and variable in their overall task performance but, most important, did not show signs of poor impulse control. Suggested short-term memory problems in these children could not be confirmed. Performance could not be attributed to early disease-related factors or lowered intelligence.