The association between plaque and caries in older children and adults has been poor, however, some studies show that there may be a relationship in younger children. The aim was to study the relationships between dental caries and dental plaque among 12-36-month-olds in Tehran, Iran. A cross-sectional study among a stratified random sample of 504 children aged one to three years from 18 public health centres in Tehran. Mothers were interviewed about their child's date and order of birth, gender, primary caregiver, the mother's age and the educational level of both parents. Dental examination was carried out according to the WHO criteria. Early childhood caries (ECC) was defined as the presence of any dmf teeth. Dental plaque was visually inspected on the labial surfaces of upper central incisors. Data analysis included Chi-square test, t -test, anova and logistic regression modelling. The prevalence of ECC ranged from 3 to 33% depending on age group, with a mean dt of 1.1 for 26- to 36-month-olds. No gender-differences existed in ECC prevalence and mean dt. Dental plaque was visible on at least one index tooth for 65-75% of the children. Presence of ECC was related to the presence of dental plaque (OR = 1.5; 95% CI 1.0-2.3) when controlling for background factors by means of logistic regression. The high occurrence of visible plaque and rather high ECC prevalence call for improvement in oral health promotion programs of the children.