Objective: The objective was to develop a Finnish Children Healthy Eating Index (FCHEI), to determine the relative validity of the index and to examine associations between the index and familial sociodemographic and lifestyle characteristics. Design: Cross-sectional samples of children participating in a population-based birth cohort study in Finland. Setting: Type I Diabetes Prediction and Prevention (DIPP) Study cohort. Subjects: Three-day food records from 1-year-old (n 455), 3-year-old (n 471) and 6-year-old (n 713) children were completed between 2003 and 2005. Results: Validity of the FCHEI was assessed by studying the associations between the FCHEI and nutrient intakes of the children. Among all age groups, intakes of SFA and sugars decreased across increasing quartiles of the FCHEI while intakes of PUFA, dietary fibre, vitamin D and vitamin E increased. Among 3- and 6-year-olds, being cared for at home was associated with the lowest FCHEI quartile (diet that deviates most from the recommendations). The lowest FCHEI quartile was also associated with residence in a semi-urban area among the 3-year-olds and low maternal education and smoking during pregnancy among the 6-year-olds. Conclusions: The FCHEI serves as a valid indicator of the quality of Finnish children's diet. Public health programmes aimed at improving the dietary behaviours of pre-school aged children should aim to improve the quality of food served at home. Families with history of lower parental education, maternal smoking during pregnancy or non-urban place of residence may require special attention.