Purpose: The purpose of this case-control study was to investigate the relationship between molar-incisor hypomineralization (MIH) and pre-, peri-, and postnatal conditions of children and mothers in South Korea. Methods: The Korean Academy of Pediatric Dentistry con- ducted this study to examine factors associated with MIH among six- to 13-year-olds. The European Academy of Pediatric Dentistry criteria and self-administered questionnaires associated with MIH were used. Results: In multivariable logistic regression analysis, the odds ratio (OR) of MIH for children whose mothers used health supplements during pregnancy was 0.65 (P=0.009). Also, children with more than three hours of out- door activities per day tended to have a lower MIH (P=0.03) than did those with zero hours of outdoor activity. Additionally, the OR of MIH for children whose mothers smoked during pregnancy was 2.37 (P=0.019) and the MIH found to be 1.33 times more frequent in children with respiratory infections during the first three years of life (P=0.048). Conclusions: Maternal smoking during pregnancy and child's pediatric respiratory infection suffered within three years after birth are factors associated with the MIH among Korean children. Further study is needed because the prevalence of MIH in children whose mothers taking health supplements (vitamins or folic acid or iron) during pregnancy is low.