Chromosomal aberrations have been evaluated in peripheral lymphocytes of four patients before and after X-ray examination with contrast medium. In each patient the rate of dicentrics was significantly increased after examination. Averaged over the patients, the increase is about seven times higher in comparison to the value before examination, an aberration frequency too high to be induced by radiation alone. Since no predisposed chromosomal radiation sensitivity was observed by in vitro experiments and the aberration yield is comparable to all patients, we assume that the increase after X-ray diagnosis is likely to be induced by application of contrast medium.