Entry into the left atrium during cardiac catheterization may be essential for full assessment of the hemodynamic situation, particularly for the accurate calculation of pulmonary blood flow and pulmonary arteriolar resistance. The retrograde transaortic transmitral technique of left atrial catheterization has been described in adults but no detailed reports are available for the pediatric age group. Experience of this technique in 43 children with congenital heart disease is now presented, with a success rate of 67 per cent and a low incidence of complications. This method compares favorably with other methods of left atrial catheterization when the interatrial septum is intact.