ABSTRACT. Objectives. More than 85% of children born today with chronic medical conditions will live to adulthood, and many should transfer from pediatric to adult health care. The numbers of adults with congenital heart defects (CHDs) are increasing rapidly. Current guidelines recommend that just over half of adult CHD patients should be seen every 12 to 24 months by a cardiologist with specific CHD expertise at a regional CHD center, because they are at risk for serious complications (eg, reoperation and/or arrhythmias) and premature mortality. The present study aimed to determine the percent of young adults with CHDs who successfully transferred from pediatric to adult care and examine correlates of successful transfer.Design. Cross-sectional study with prevalence data from an entire cohort.Setting and Patients. All patients (n ؍ 360) aged 19 to 21 years with complex CHDs who, according to current practice guidelines, should be seen annually at a specialized adult CHD center were identified from the database of the cardiology program at the Hospital for Sick Children in Toronto, Canada, a pediatric tertiary care center. Of these patients, 234 completed measures about health beliefs, health behaviors, and medical care since age 18 years.Main Outcome Measure. All 15 specialized adult CHD centers in Canada formed the Canadian Adult Congenital Heart (CACH) Network. Attendance for at least 1 follow-up appointment at a CACH center before the age of 22 years was ascertained for all eligible patients. Attendance at a CACH center provides a clear criterion for successful transfer.Results. In the total cohort, 47% (95% confidence interval [CI]: 42-52) had transferred successfully to adult care. There was no difference in rates of successful transfer between patients consenting to complete questionnaires (48%) and those who declined (47%). More than one quarter (27%) of the patients reported having had no cardiac appointments since 18 years.In Conclusions. This is the first study to document the percent of young adults with a chronic illness who successfully transfer to adult care in a timely manner. Patients were from an entire birth cohort from the largest pediatric cardiac center in Canada, and outcome data were obtained on all eligible patients. Similar data should be obtained for other chronic illnesses. There is need for considerable improvement in the numbers of young adults with CHDs who successfully transfer to adult care. At-risk adolescents with CHDs should begin the transition process before their teens, should be educated in the importance of antibiotic prophylaxis, should be contacted if a follow-up appointment is missed, and should be directed to a specific CHD cardiologist or program, with the planned timing being stated explicitly.Adult care needs to be discussed in the pediatric setting, and patients must acquire appropriate beliefs about adult care well before transfer. Developmentally appropriate, staged discussions involving the patient, with and without parents, throughout adolescence may help pa...