For the purpose of investigating the etiologic factors of early childhood caries in 18-month old infants which is an important period in pediatric dentistry, 234 infants of 18-month age and their first care-givers (mothers in most cases) who visited Ilsin Christian Hospital in Busan for the baby-care counseling and vaccination were examined. The oral examination was performed on the infants and their care-givers were individually interviewed about various items, and the data was statistically processed to yield the results as follows : 1. The prevalence rate of early childhood caries was 27.4%. 2. The percentage of the eldest child, the education level of the mother and monthly income were revealed lower in caries group infants. 3. The percentage of prolonged breast-feeding and the frequency of daily nursing was higher in early childhood caries group. 4. In early childhood caries group, the between-meal snacks were tend to be provided irregularly without predetermined time, and sugar containing snacks such as chocolates and candies were ranked in higher order whereas the fruits and milks were in lower ranks. 5. In early childhood caries group, the percentage of those performing the oral hygienic care was lower and the timing to start the toothbrushing was comparatively later. From the above results, it can be concluded that prolonged breast-feeding, poor snack habits and lower socioeconomic status act an important role in early childhood caries in 18-month old infants.