During a 24-year period (1963-1987), 46 infants and children with complete transposition of the great arteries (CTGA) and an essentially intact ventricular septum (IVS) have undergone surgical relief of left ventricular outflow tract obstruction (LVOTO) concurrently with the Mustard operation (MO). The obstruction was valvar in 2 patients, subvalvar in 43 and combined valvar and subvalvar in 1; the ventricular septum was intact in 42, and 4 patients had a small VSD at the time of repair. Neonatal and pre-Mustard cardiac catheterization (CC) data were analyzed to obtain LV to right ventricular peak systolic pressure ratio (LV/RVPSP) and LVOT peak systolic gradient (PSG). The mean neonatal LV/RVPSP (n = 31) was 0.79 +/- 0.04; pre-Mustard LV/RVPSP (n = 44) was 0.75 +/- 0.05; pre-Mustard LVOT PSG (n = 34) was 46.5 +/- 3.5 mmHg. Neonatal (n = 29) and pre-Mustard (n = 30) cardiac angiograms were retrospectively reviewed to analyse morphologic substrates of LVOTO. In the neonatal period, LVOTO was present in 7 patients (anatomic, n = 4; dynamic, n = 2; combined, n = 1). Immediately prior to the MO, LVOTO was present in 29 (anatomic, n = 20; dynamic n = 2; combined, n = 7). Techniques to correct LVOTO at the initial MO included pulmonary valvotomy (n = 3), ventriculomyectomy (n = 41), resection of an windsock aneurysm (n = 1), LV to pulmonary artery valved conduit (VC) (n = 1). Two patients underwent VC early postoperatively (PO) for severe residual LVOTO (both died).(ABSTRACT TRUNCATED AT 250 WORDS)