Background and Research Objectives: Most children with congenital heart disease (CHD) are expected to survive to adolescence and adulthood owing to medical advances and care management. These adolescents need to be well informed about their exercise capacity and take greater personal responsibility for their exercise behavior as they mature. The aims of this study were to compare the amount and intensity of exercise engaged in by male and female adolescents with mild CHD while on summer vacation and during the academic semester and to determine the extent to which their exercise behavior met cardiologists' recommendations, based on New York Heart Association functional classification. Participants and Methods: A repeated-measure design was used to evaluate exercise behavior in 126 adolescents 12 to 18 years old with mild CHD from the outpatient cardiology departments of 3 medical centers in Taiwan. Exercise, classified as mild, moderate, or vigorous, was evaluated during summer vacation and during the fall semester using a 7-day self-reported exercise log. Results and Conclusions: Patients engaged in significantly more mild and total exercise during summer vacation than they did during the fall semester. They also engaged in significantly less vigorous exercise during summer vacation than they did in the fall semester. Female respondents engaged in significantly less moderate (P = .019), vigorous (P < .001), and total (P = .015) exercise than did their male counterparts but showed no difference in mild exercise. During the summer and fall, nearly 50% of the adolescents followed their cardiologist's recommendations for exercise. Adolescents with mild CHD engaged in more exercise during summer vacation but engaged in more vigorous exercise during the fall semester. Approximately one half did not follow the exercise intensity recommended by cardiologists. Inadequate exercise patterns may lead to cardiovascular complications. Planned interventions related to exercise instruction are needed for adolescents with CHD.