['Physical fitness is reduced in adults with Down syndrome (DS). The present study was conducted to elucidate the exercise response in adults with DS.', 'Case controlled before-after trial.', 'Residential centre for people with intellectual disabilities.', '96 Adults with DS, 25 non-DS adults with an intellectual disability, 33 controls.', 'Echocardiography to exclude heart defects and to measure cardiac index (CI) in the supine position, supine position with raised legs, and following ten knee bends.', 'Exercise testing', 'At rest, mean CI was not significantly different between persons with DS and controls (2.3 vs. 2.4\xa0l/min/m(2), p\u2009=\u20090.3). However, mean CI after exercise was significantly lower in DS (2.9 vs. 3.7\xa0l/min/m(2), p\u2009<\u20090.001) and mean CI increase from rest to exercise was more than 50% lower in DS. On the contrary, CI after exercise was similar among controls and non-DS adults with an intellectual disability. Significantly lower stroke volumes in DS were found with insufficient heart rate response.'] Do adults with Down syndrome have reduced cardiac response after light exercise testing?