The development of pulmonary arteriovenous malformations is a well-known complication after Fontan operations, and may result in significant morbidity due to increasing arterial desaturation. We compared the use of bubble contrast echocardiography and pulmonary angiography in detecting such malformations. We also examined which anatomical and haemodynamic variables were associated with their development. Our study includes 20 patients who had undergone modified Fontan procedures, 10 with atriopulmonary and 10 with total cavopulmonary connections, in Gothenburg between 1980 and 1991. All patients underwent cardiac catheterisation and pulmonary angiography. Bubble contrast echocardiography was performed at the same time, with injection of agitated polygelin colloid solution (Haemaccel, Hoechst) into the right and left pulmonary arteries, respectively. Transoesophageal echocardiography was used to detect the appearance of bubble contrast in the pulmonary venous atrium. The aim was also to evaluate the role of hepatic venous blood. Of the 20 patients, 9 (45%) had a positive contrast echocardiography study, compared with only 2 (10%) detected by pulmonary angiography. Patients with positive contrast echocardiography had a significantly lower arterial oxygen saturation than those with negative studies, both at rest (88% vs 95%, p < 0.01) and during exercise testing (78% vs 89%, p = 0.01). Bubble contrast echocardiography is much more sensitive in detecting pulmonary arteriovenous malformations than pulmonary angiography. By injecting echo contrast into the right and left pulmonary arteries, the method can be made highly selective. Pulmonary arteriovenous malformations develop much more frequently in patients with the Fontan circulation than previously reported.