The 12 mm Dacron conduit containing a porcine valve is the smallest valved conduit manufactured and is used in the youngest infants with the most diminutive pulmonary arterial system. The outcome of patients with such a conduit is unknown. Between 1975 and 1985 there were 49 hospital survivors after placement of a 12 mm extracardiac valved conduit from the right ventricle to the pulmonary artery. Follow-up is available in 42 patients, aged 1 to 16 months (mean 3.5) and weighing 2.5 to 8.7 kg (mean 3.8). Twenty-eight patients (67%) have undergone subsequent conduit replacement, and 11 (26%) are alive and asymptomatic with a mean follow-up of 56 months. There were three late deaths. The interval between implantation and conduit change was 4.5 to 101 months (mean 44), allowing a weight gain of 2.7 to 23 kg (mean 10.4) before reoperation at age 12 to 117 months (mean 49). Despite elevated right ventricular pressures equaling systemic values, 37% of these patients were clinically asymptomatic. The gradient across the 12 mm valved conduit before explantation ranged from 30 to 173 torr (mean 83) with an almost equal predilection for stenosis at the proximal anastomosis, valve, conduit, distal anastomosis, and main pulmonary artery. The intervening pulmonary artery growth determined the size of the replacement conduit, 14 to 25 mm (mean 16), and was the main factor influencing the results of reoperation. This study demonstrates that the 12 mm porcine valve-containing conduit affords palliation in this difficult subset of patients with the smallest pulmonary arterial tree.