: Authors present the long-term development of two monozygotic twin sisters. One of them was detected by newborn screening as athyreotic (gemelus A) and healthy gemelus B sister. Therapy in athyreotic girl started at 4 weeks after birth and was monitored in order to maintain serum thyroxine and TSH in normal range. Both sisters spent their chilhood and adolescence together in the same family, school and work together as tailors in the same factory. Their development was very similar. In order to detect subclinical difference of their development, the school achievements were tested using their school certificates. Studies were divided to testing mechanical memory, logical thinking and skills. The evaluation of all classes of school attendance (13 years) reveals subtle differences - deficit of patient at approximately 6-15% level, mainly in studies demanding skills. However, dynamic evaluation after years reveals that the worse results of the patient (gemelus A) occurred during the first years of basic school whereas during adolescence having attended the special school tailor, she attained better results than her healthy sister. It seems that mental deficit in early treated hypothyroid patient is not severe and varies even during the life. Problem of the fetal hypothyroidism is probably more related to the iodine deficiency, which assures necessity of thyroid hormones for athyreotic child from mother's or twin's normally supplemented thyroid glands.