A study of "deep rectal temperature" was performed in 25 unwarmed infants less than 2 years of age undergoing cardiac catheterization. All 13 children (including 3 unsedated infants) whose body temperature fell 2° C or more were 6 months of age or less. Temperatures dropped 1.1° C or less in the remaining 12 patients, all of whom were 6 months of age or more. In general, those children who suffered the greatest falls were smaller, less well nourished and had larger surface areas relative to body mass. In addition, arterial desaturation was found more often in these patients. Hypothermia and its complications have been prevented in 16 infants undergoing cardiac catheterization with the assistance of a safe, simple warming device. The young patients warmed were similar to the 13 unwarmed infants found to develop hypothermia in environmental conditions comfortable for laboratory personnel. Not only is hypothermia in infancy a hazard per se, it is without apparent metabolic advantage. Furthermore, the alterations in oxygen consumption that occur with cold in these small patients may invalidate any calculations that assume stable oxygen consumption. Therefore, it is concluded that hypothermia should be avoided in the young infant studied in the cardiovascular laboratory. The warming system presented in this report is suggested as a method with which this undesirable condition may be prevented.