To assess the clinical and functional status of a young Down syndrome (DS) population without congenital heart disease. Prospective study of children with DS and control subjects. Bambino Gesù Children's Hospital, Rome, Italy. Forty-two children with DS (mean +/- SD age, 9.8 +/- 3.6 years) underwent genetic, clinical, neuropsychological (IQ), and cardiorespiratory evaluation. Cardiorespiratory fitness was assessed with a treadmill test and a lung function test to determine forced vital capacity, first-second forced expiratory volume, and peak expiratory flow. Data were expressed as a percentage of the predicted values for control children. To assess cooperation during exercise testing, we devised a compliance scale assigning a score according to the subjects' ability to understand instructions, ability to walk and run as required, and need for vocal encouragement. Eighteen (43%) of 42 children with DS were obese, 10 (24%) were short, and 17 (40%) had microcephaly. On the Leiter International Performance Scale, 2 of 35 subjects had a normal IQ score (80-120); all others had low IQ scores (79 to <40). Five subjects did not undergo cardiorespiratory assessment. Eighteen of 37 subjects completed lung function tests: the results for 10 were unremarkable, and results from 8 revealed reduced forced vital capacity because of poor compliance. The subjects tested had low exercise tolerance (mean +/- SD tolerance percentage, 61% +/- 12%), mild tachycardia (maximal heart rate, 91% +/- 4%), and a mild hypertensive response (maximal blood pressure, 89% +/- 8%). Compliance scores correlated significantly with exercise time and age but not with IQ. Clinical and cardiorespiratory assessment is feasible in subjects with DS without congenital heart disease and should be useful in gauging their fitness level for safe physical activity.