An important component of high-dose chemotherapy/autologous bone marrow support regimens for adjuvant treatment of breast cancer is carmustine. Preclinical studies have shown that the level of the DNA repair protein O6-methylguanine-DNA methyltransferase is correlated with the resistance of cultured human tumor cells to this drug, but little is known about transferase levels of breast tissue in vivo. We measured the DNA repair activity in 80 tissue samples from 65 patients, including normal, abnormal, benign, and malignant specimens. Wide interindividual variations was observed and average transferase levels were similar in normal and benign tissue. However, transferase levels were significantly elevated in stage I-IV disease. In addition, the frequency of samples with no detectable transferase was greatly reduced in this malignant group, and transferase was positively correlated with the presence of positive nodes, a marker for disease progression. In contrast, transferase levels were not correlated with age or estrogen receptor status, and the levels in normal tissue did not vary between patients with benign or malignant disease. These results suggest that this DNA repair activity may be increased in breast cancer relative to normal tissue and encourage further study of the predictive value of transferase measurements in high-dose chemotherapy/autologous bone marrow transplant for breast cancer.