The Japan Environment and Children's Study is a birth cohort study designed to identify environmental factors that affect children's health. In particular, an elucidation of the effects of chemical exposure during the fetal and neonatal periods is the main objective of the study. The Study has been planned and is fi nanced by the Ministry of the Environment of Japan. In addition, an awareness among the developed countries of the importance of children's health with respect to environmental chemicals has accelerated the planning.