Obesity during childhood is a matter of growing concern and needs to be continuously monitored in respect of its importance to public health. We examined the prevalence of obesity and its development trends in preschool and school children living in the administrative district of Hyogo prefecture in Japan. The cross-sectional study analyzed routinely collected data, including measurements of height and weight, from between 160, 000 and 530, 000 school children during the period 1981-2000 and from between 46, 000 and 66, 000 preschool children aged 4 and 5yrs during the period 1987-2000. Obesity is defined as being greater than 120% of the standard body mass for height and gender based on the criterion used in National Health and Nutrition Survey of Japan. The prevalence of obese preschool children aged 4 and 5 yrs has increased by 1.5 times from 2.2% to 3.0% during the last 14 years, while that of obese school children has increased nearly 2.5 times from 3.9% to 9.3% during the last 20 years. The recent prevalence of obesity has exceeded 10% in older than third graders. The frequency of obesity tended to increase when the children reached the first and/or third graders. Our data suggest that childhood obesity is maintaining an upward trend, and further imply that preventing and treating obesity beginning at the preschool age is a crucial issue in pediatric public health.