A selected group of 18 patients aged 15-27 years with transposition of the great arteries and a previous Mustard procedure were evaluated to determine their functional ability and clinical state. Arrhythmias were common, occurring at some time in 16/18 (89%). Arrhythmia was serious in four; two of them required pacing and two had cardiac arrests, one resulting in death. Seven (41%) had right ventricular dysfunction; this was progressive in three. Tricuspid regurgitation was present in seven (41%); it occurred in patients with normal and reduced right ventricular ejection fractions. Regurgitation became progressively worse as the right ventricle dilated. Left ventricular function was well preserved in most patients. Fourteen (82%) of this pioneer group were leading normal lives (ability index 1 or 2). Although these results are acceptable concern remains about the probability of deteriorating right ventricular function.