Stability of genome of children born to patients after antitumor radio- and chemotherapy was studied. For this aim the peripheral lymphocytes were irradiated in vitro at doses 0, 0.25, 0.50, 1.00, and 1.50 Gy of gamma-rays 137Cs, and induced chromosome aberrations on metaphases of cultivated lymphocytes were scored. The data obtained demonstrate an increased chromosomal radiosensitivity of patients' children as compared to the control children. A possible association between the chromosomal instability and the increased risk of cancer is discussed. The children prezygotically exposed to mutagenic factors are supposed to make a group of potentially increased risk of cancer.