The purpose of our paper is to assess the prognostic value of balloon occlusion as a test for patients with various types of functionally single ventricle after different palliative surgical procedures who were candidates for further modifications of a cavo-pulmonary operation. In all patients, there were at least two sources of pulmonary arterial supply.Our hypothesis was that, using balloon occlusion temporarily to produce changes in the haemodynamic situation, we could simulate the potential state subsequent to surgical palliation. We used balloon occlusion in 19 patients during diagnostic catheterisation, followed by interventional and or surgical procedures as considered necessary. This test was applied for temporary closure of the pulmonary valve in 8 patients, and a Blalock-Taussig anastomosis in 11 patients.