Objectives The aim of the present survey was to study the association between dental caries and body mass index (BMI) in Swedish preschool children (born in 1999). Methods A population‐based and cross‐sectional study design was used comprising all 920 5‐year‐old children in a defined area in and around the city of Lund. Anthropometric measures for the calculation of BMI were retrieved for each child from recordings at Child Health Care Centers (CHC). The occurrence of caries and fillings in the primary dentition, defined as deft (decayed, extracted, or filled primary teeth) and dt (decayed primary teeth), was collected from the children's dental records. Results The mean BMI was 16.1 (no differences between boys and girls). About 19.2% were overweight, of which 5.1% were obese. Overweight or obese children did not have higher deft or dt than others. However, children with low BMI (below −1 SD of national mean values for Swedish 5‐year‐olds) had statistically significantly higher deft and dt than children with normal BMI. Conclusions Children with low BMI may be at risk of caries development. Low BMI may be associated with eating habits endangering dental health.