Nineteen children treated for Wilms' tumor (thirteen cases) or Hodgkin's disease (six cases) with cytostatic agents and/or radiotherapy were studied cytogenetically on lymphocytes cultivated from blood samples drawn after at least 1 year of complete remission after end of therapy. A reference group of children was matched for age, sex, and residence. The frequencies of sister chromatid exchange (5.4 versus 5.6 SCE/cell), and chromosome damage type gaps (6.6 versus 7.1%) and breaks (1.9 versus 1.9%) were not different in the two groups, but exchange type aberrations were more frequent in the patients (0.9 versus 0.06%). Fifty karyotypes were analyzed in all but two cases of Hodgkin's disease. The overall frequency of stable (3.1 versus 3.8%) and unstable (1.7 versus 1.4%) structural chromosome changes such as translocations, deletions, chromatid exchanges, and dicentrics were not different in the patient and the control groups. If the chromosome data reflect a general cancer risk, this risk cannot be considerably higher among the cancer-treated children.