Chromosome damage (CD) and sister chromatid exchange (SCE) levels were studied in lymphocytes from 30 pediatric heart catheterization patients receiving radiation during diagnostic fluoroscopy and cineangiography procedures. Forty-eight-hour CD and 72-hr SCE cultures were prepared from sequential samples taken from each patient: samples 1-3 via the catheter the same day (1) before exposure, (2) after fluoroscopy, and (3) after cineangiography; and sample 4 by venipuncture the next morning. Significant increases in CD (dicentrics, rings, and fragments), but not SCE, were observed. From a mean base level of 0.4% cells with CD, the CD levels increased 2-3-fold in samples 3 and 4 (p = .001). Rings only occurred in samples 3 and 4. While increased CD levels also correlated with increasing age, body surface area, and weight, partial correlations controlling for these factors clearly indicate that the CD effects are principally attributable to the radiological procedures (p = .001). Increased CD levels correlated with both the roentgen dose of cineangiography exposure (p = .002) and the volume of contrast medium (p = .000); however, partial correlations, controlling for either factor, indicate that the contrast medium was the principal factor (p = .006).