["Journal of Intellectual Disability Research, Volume 70, Issue 5, Page 491-499, May 2026. ", "\nABSTRACT\n\nBackground\nAdults with Down syndrome (DS) are less physically active than those without DS. Among adults with DS, those with congenital heart disease (CHD) have lower physical activity than those without CHD. Many trials exclude people with CHD from exercise trials; thus, the safety and effectiveness of these interventions for increasing physical activity and fitness in adults with DS and CHD are not known. The purpose of this analysis was to evaluate the safety and preliminary effectiveness of an exercise intervention in adults with DS and CHD for increasing physical activity and fitness.\n\n\nMethod\nThis secondary analysis used data from a 12‐month randomized controlled physical activity intervention for adults with DS. Safety of the intervention was assessed as number and severity of adverse events and compared by CHD status. Changes in physical activity (accelerometry) and fitness (VO2peak) among those with CHD were evaluated using mixed effects models.\n\n\nResults\nThirty‐six participants had CHD and were randomized to one of the exercise intervention arms (average age 25.5 years, 55.6% female). There were no differences in number (CHD: 18 vs. no CHD 22 events; rate ratio 1.02, p = 0.94) or severity (p = 0.25) of adverse events between those with and without CHD. Participants significantly increased moderate‐to‐vigorous physical activity minutes per day (+8.6 min/day, p = 0.045) and VO2peak (+2.1 mL/kg/min, p = 0.036).\n\n\nDiscussion\nResults from this analysis show preliminary support for young adults with DS and CHD to safely participate in exercise interventions that are appropriately designed for their inclusion. These interventions also demonstrate preliminary effectiveness for increasing physical activity and fitness. Findings should be repeated and confirmed in a larger, more diverse sample to understand the safety and impact of exercise on health in persons with DS and CHD.\n\n"]