The aim of this study was to assess the performance of pediatric-size oximetry catheters in diagnostic heart catheterization. Measurements of pressures and oxygen saturations were obtained with oximetry catheters as well as with standard methods during routine heart catheterizations in 10 infants and children with congenital heart defects. Maneuverability of the oximetry catheters was comparable to that of standard Swan-Ganz catheters. Pressure tracings were damped, but mean pressures were undistorted. Oxygen saturation measurements exhibited bias and scatter. It is concluded that no advantage is obtained by the use of oximetry catheters in diagnostic heart catheterization in children and infants.