Di (2-ethylhexyl) phthalate (DEHP) is commonly used phthalates and concerns of adverse effects of prenatal DEHP exposure on neonatal thyroid hormone (TH) and neurodevelopment are increasing. However, there is no report regarding association between prenatal DEHP exposure and infant neurodevelopment including TH levels in Japanese population. Thus the aim of present study was to evaluate the associations between prenatal DEHP exposure and mental and psychomotor development of infants 6 and 18 months along with investigating influence on neonatal free thyroxine (FT4) and thyroid stimulating hormone (TSH) levels in the prospective birth cohort study. Maternal blood samples collected between 23 and 41 weeks of gestation was analyzed for mono (2-ethylhexyl) phthalate (MEHP), metabolite of DEHP levels. Neonatal FT4 and TSH were obtained from mass screening data. Infant neurodevelopment was assessed by Bayley Scale of Infant Development second edition at 6 and 18 month of age. For the final analysis, 328 participants were included. The median levels of maternal MEHP was 10.6 ng/ml, neonatal TSH and FT4 was 2.20 mu U/ml and 2.03 ng/ml, respectively. We did not find any associations between prenatal DEHP exposure and neonatal TH levels or infant mental and psychomotor development at 6 and 18 month. In this study, prenatal DEHP exposure did not showadverse effects on infant TH levels or mental and psychomotor development in early life stage. However, our previous study revealed negative effects of prenatal DEHP exposure on sex hormone levels, continuous investigation on neurodevelopment in later life in association with prenatal DEHP exposure is necessary. (C) 2016 Published by Elsevier B.V.