The ability of first pass radionuclide angiocardiography to detect and quantitate residual intracardiac shunts and systemic venous obstruction after repair of transposition of the great arteries was evaluated in 29 children. Information from radionuclide scans was compared with data obtained during cardiac catheterization. Three children had a residual right to left shunt detected with both methods. There was good agreement between radionuclide and catheterization quantitation of left to right shunt in the nine patients with a residual defect, five of whom had significant shunting. Nineteen patients with signs suggesting superior vena caval obstruction were evaluated with both radionuclide and catheterization methods. In eight, complete obstruction was detected with both techniques; in one additional patient, partial obstruction was found on catheterization only. One of six patients evaluated for possible inferior vena caval obstruction was identified with both techniques. In the group as a whole, information obtained with radionuclide angiography correlated well with cardiac catheterization data in evaluation of residual shunts and obstruction to systemic venous return.