['To determine the changes in percentage of body fat, cardiovascular fitness, and insulin levels during the 3-month summer break in overweight children enrolled in a school-based fitness program.', 'Overweight middle-school children were randomized to a lifestyle-focused physical education class (treatment) or standard physical education class (control) for 1 school year (9 months; previously reported). This analysis reports changes during the 3-month summer break in children who participated in the fitness intervention group and who remained at this school the following year and repeated a fitness class. At the beginning and end of the school year, children underwent evaluation of (1) fasting levels of insulin and glucose, (2) body composition by means of dual x-ray absorptiometry, and (3) maximum oxygen consumption as determined by treadmill use.', "Rural middle school and an academic children's hospital.", 'Overweight middle-school children.', 'School-based fitness curriculum, followed by summer break, and an additional year of school-based fitness intervention.', 'Cardiovascular fitness test results (maximum oxygen consumption), body composition, and fasting insulin levels.', 'Improvements seen during the 9-month school-year intervention in cardiovascular fitness, fasting insulin levels, and body composition were lost during the 3-month summer break. During this summer break, mean +/- SD fitness level decreased (maximum oxygen consumption, -3.2 +/- 1.9 mL/kg per minute; P = .007), fasting insulin level increased (+44 +/- 69 pmol/L [+6.1 +/- 9.7 mIU/mL]; P = .056), and percentage of body fat increased (+1.3% +/- 1.3%; P = .02) to levels that were similar to those seen before the school intervention.'] Are school-based fitness changes lost during the summer vacation?