Routine use of the hydrogen platinum electrode system (HPES) in 1,375 consecutive cardiac catheterizations facilitated shunt detection in 171 patients, led to the diagnosis of unsuspected shunts in 18 patients, and was of particular value in accurately localizing unusual left-to-right shunts. Left-to-right shunts were detected in 171 patients (12% of the total catheterizations). Eighteen unsuspected shunts (10% of the total shunts, or 1% of the total catheterizations) were found. Atrial septal defect (ASD), partial anomalous pulmonary drainage (PAPVD), ventricular septal defect (VSD), and ruptured Valsalva aneurysm were demonstrated, usually associated with other congenital or acquired lesions. HPES was of particular value in accurately localizing the venous drainage of 38 unusual or complex lesions with left-to-right shunting. Five patients with rheumatic mitral valve disease had PAPVD, a higher incidence (1.2% of rheumatic mitral valve disease) than had previously been reported. The rationale for use of a sensitive shunt detection method as a routine part of the catheterization procedure is discussed.