OBJECTIVES: Chile is a unique example of the nutrition transition, with improvements in undernutrition and subsequent rapid increases in obesity occurring at a rate much faster than in other countries. This study aims to describe the cross-sectional obesity trends in Chilean preschool children from 1996 to 2004. MATERIALS AND METHODS: Anthropometric measurements from preschool children (ages 2-5) registered in the Junta Nacional de Jardines Infantiles program from 1996 to 2004 were analysed. The weight-for-height z score using the World Health Organization 1977/1985 reference curves for age, sex, height, and weight were defined to estimate overweight and obesity. The overall trends in obesity were described for each study year and generalized estimating equation models were used to describe trends in obesity after adjusting for the correlated nature of repeated measurements in individual children in consecutive years. RESULTS: The prevalence of obese and overweight children has remained constant in both boys and girls during the past 9 years; however, after adjustment for repeated measurements in individual children, the odds of obesity increased by 30% each year (odds ratio [OR] 1.28, 95% confidence interval [CI] 1.27-1.31, P < 0.001). Furthermore, the odds of obesity were significantly greater in both August and November compared with March. CONCLUSIONS: The prevalence of obesity in Chilean preschool children continues to be a major public health concern, and cross-sectional trends may underestimate the magnitude of the problem.