Chromosomal aberrations in lymphocytes were studied in patients with untreated cancers of the lung and skin (basal cell carcinoma) and in controls matched for sex, age, and smoking habits. Patients with skin cancer were found to have a significantly increased rate of chromosome type aberrations. Among the patients with lung cancer, there was a tendency towards an increased rate of aberrations which, however, did not reach statistical significance. The results suggest that an increased rate of chromosomal aberrations in cultured lymphocytes is an indicator of genetic damage which may increase the cancer risk.