Chromosome aberrations were studied in peripheral blood lymphocytes from untreated testicular cancer patients and others treated with chemo- and/or radiotherapy. A distinct increase in spontaneous aberrations over the level of healthy controls was found in patients treated with surgery alone. Our data suggest the existence of a certain degree of chromosome instability which may be a factor in the development of malignancy for testicular tumours, too. The frequency of aberrant cells was much higher in treated groups than in controls, and the total of aberrations was therapy related. The frequency of aberrant cells was the highest in the first 2 years after the end of treatments similarly to the results of 3 serially examined individuals. The decrease in aberrant cells was time-dependently gradual only in X-ray-treated patients. Real conclusions about the nature of therapy-related persistence of aberrant cells can be drawn from the study of a sufficient number of testicular cancer patients studied more than 1 year after the end of treatments.