Background: Congenital hypothyroidism is the most common reason of mental retardation, and normal neurological development can be provided by early and effective treatment. In this present study, it is aimed to compare neurological developments of patients in 6-42 months of age with congenital hypothyroidism and healthy controls of the same age group prospectively by Bayley III test. Methods: In this present study, neurological developments of 41 congenital hypothyroidism cases and 39 healthy controls, who applied to Pediatric Endocrinology Section of Mersin Children Hospital and Pediatric Neurology Outpatient Clinic of the Medical School at Mersin University between years 2009 and 2011, were evaluated by Bayley III test. Results: Cognitive, language and global motor scores in addition to receptive communication, expressive communication, fine motor and gross motor subscores in children with congenital hypothyroidism were statistically significantly, lower than those in the control group (p < 0.05). It is detected that initiation dose and day of treatments, severity of hypothyroidism and time to normalization thyroid stimulating hormone had no statistically significant effects on neurological development of the study group (p > 0.05). In both groups, as the education levels of mothers are increased, language development scores are also increased (p < 0.05). Additionally, statistically significant increases in Bayley III scores except cognitive scores have been observed in both groups as the level of income is increased (p < 0.05). Conclusions: Despite early and effective treatment in newborns with congenital hypothyroidism, retardation in neurological developmental has been detected. This situation can be related to influences on neurodevelopment in intrauterine period. According to our present knowledge, this study is the first case control study in the literature that neurological developments of congenital hypothyroidism patients are evaluated with Bayley-III score. (c) 2012 Published by Elsevier B.V.