Background: In Chile, obesity is currently the main nutritional problem. Since prevention should start early in life, it is important to determine the prevalence of obesity during childhood according to age category. Aim: To determine and compare the evolution of the obesity prevalence and other anthropometric indicators in preschool children between 2002 and 2004. Material and methods: Twice a year, we analyzed the data of children aged 2 to 4 yrs, from day care centers belonging to the National Association, of Day Care Centers located in Greater Santiago, from 2002 till 2004 (the number of children included on each point in time fluctuated between 3,500 and 10, 000). Cross-sectional and longitudinal analyses were carried out to determine the evolution of obesity prevalence, weight for age (WA) and body mass index (BMI) Z scores (according to the Centers for Disease Control 2000 reference) on preschoolers who were 2 years old in March 2002 and that were followed 3 yeas, until November 2004. These parameters were compared by age and gender over time. Results: The prevalence of obesity varied between 11 and 13.6% in two-year old children and between 17% and 20% in three and four year olds. The cross-sectional analysis showed that WA and BMI Z scores were significantly lower at 2 years of age, while the longitudinal analysis clearly demonstrated that there was a sharp rise in obesity between 2 and 3 years of age. Conclusions: The prevalence of obesity is high in preschool children, especially among the 3 and 4 year-olds with a significant rise from 2 to 3 years of age.