Current diagnostic cardiac studies in infants and children result in longer x-ray exposure times and in more angiocardiograms per patient. Blood samples removed before and immediately after such studies in 20 infants and children have shown chromosome damage in all. The damage was equal to an in vitro absorbed dose in the range of 20 to 50 rads. Since the effect was considerably greater than that calculated from the x-ray exposure dose to the patient, it is concluded that the damage is mainly due to the contrast agent used in angiocardiography. Long-term follow-up studies on such patients are indicated.