UV-C light induced DNA-repair synthesis was studied in unstimulated lymphocytes of 41 female patients (aged 33-83 years) with invasive breast cancer and of 27 healthy women (aged 37-68 years). As a parameter for the determination of the DNA-repair synthesis the incorporation of [3H] thymidine was taken in the presence of 2 mM hydroxyurea (HU). The [3H] thymidine incorporation levels were reduced in 20 of the 41 patients and in only 3 of the 27 controls. This difference between the two groups was significant. An impaired DNA-repair synthesis might be involved in the etiology of breast cancer in some patients.