The results of this work show a higher level of 8-hydroxy-2'-deoxyguanosine (8-OH-dG), a typical biomarker of oxidative stress, in uterine myoma tissues than in their respective tumor-free tissues. The level of this modified base was elevated in uterine tissues of premenopausal women when compared with postmenopausal ones. We have also found the correlation between the size of the tumor and the amount of 8-OH-dG. These results suggest that estrogen-produced 8-OH-dG may be one of the factors responsible for the formation of the myoma, and it may contribute to malignant transformation of myoma cells.