To demonstrate the use of transthoracic contrast echocardiography in the detection of pulmonary arteriovenous fistulas in patients with a previously constructed anastomosis between the superior caval vein and the right pulmonary artery (Glenn shunt), and to examine their prevalence in this special population, we evaluated prospectively all patients followed up in the Grown-Up Congenital Heart Unit subsequent to construction of a classical or bi-directional Glenn shunt. We studied 12 patients, aged from 21 to 38 (mean 28 ± 4.8) years who had had a previous cavopulmonary shunt in place for a period of 4 to 33 years (mean 24±9 years). All were examined with cross-sectional contrast echocardiography, 11 patients had cardiac catheterisation and angiography, and 6 patients had magnetic resonance imaging. Systemic arterial oxygen saturations at rest, and during exercise using the modified Bruce protocol, were measured in all patients. Contrast echocardiography showed evidence of pulmonary arteriovenous fistulas in 7 of the 12 patients, with appearance of echo contrast in the left atrium 1–8 seconds after peripheral venous injection in the arm. Simultaneous appearance of microbubbles in the right atrium revealed a residual communication between the superior caval vein and the right atrium in 2 patients, and presence of collaterals between the superior and inferior caval veins in one. Cardiac catheterisation and angiography showed obvious fistulas in 4 patients, and revealed suggestive findings in 2. In patients deemed to have pulmonary arteriovenous fistulas on contrast echocardiography, arterial oxygen saturations at rest (51–94%, mean 75±15.3%) and on exercise (23–91%, mean 53±24.2%) were significantly lower compared to patients judged to be without fistulas (p<0.005). Pulmonary hypertension in the contralateral lung was more common in patients with fistulas (mean left pulmonary arterial pressure 22–110 mm Hg, p=0.014). In patients with cavopulmonary anastomoses, pulmonary arteriovenous fistulas occur frequently in the long term (10–33, mean 25.7±8 years), and are associated with worsening systemic arterial desaturation. Contrast echocardiography should be included in the regular evaluation of these patients as a simple and sensitive technique for the detection of pulmonary arteriovenous fistulas, particularly with the devel opment of increasing cyanosis.