The DNA repair synthesis induced by UV-C irradiation was studied in unstimulated lymphocytes of 64 healthy women whose mothers, or sisters, or mothers and sisters had had breast cancer. For comparison we took 48 control women. As the parameter for the determination of DNA repair synthesis the incorporation of [3H]thymidine in the presence of 2 mM hydroxyurea was taken. The levels of [3H]thymidine incorporation were reduced by 19 of the 29 women whose mothers, in 17 of the 25 women whose sisters and in nine of the 10 women whose mothers and sisters had had breast cancer. By comparison a decreased level was found in only seven of the 48 control women. This difference between the controls and women having first degree relatives with breast cancer was significant in each group. In an earlier study a reduced DNA repair synthesis in breast cancer patients was established. The present findings suggest that DNA repair synthesis may be one of the factors involved in the genesis of breast cancer.