Chromosomal aberrations were studied in metaphases from peripheral blood cultures of 52 women with cancer of the cervix uteri, 89 cases of various grades of cervical precancerous lesions, and 47 age-matched normal (control) women. The frequency of metaphases with chromosome and chromatid aberrations was 17.24% in cancer patients, 10.41% in those with precancerous lesions, and 6.39% in control women. There was a significant (p less than 0.001) increase in the frequency of chromosome aberrations in patients with cervical precancerous and cancerous lesions, compared with controls. After the exclusion of the treated cases, cancer patients also revealed a highly significant (p less than 0.001) increase in the frequency of chromosome aberrations, compared with controls. The results of the present study indicate the existence of chromosomal instability in the majority of cervical cancer patients and in some cases of precancerous lesions. The increased frequency of spontaneous chromosome aberrations in patients with precancerous lesions may be of importance for the understanding of their biological behavior.