To better understand the molecular mechanisms of carcinogenesis induced in uterine endometrium by therapeutic anti-estrogenic Tamoxifen (TAM) exposure, 27 uterine tumors (4 benign endometrial polyps and 23 carcinomas) associated with TAM exposure were analyzed for the presence and spectrum of p53 and K-ras mutations. Although there was no significant difference between TAM-associated endometrial carcinomas and sporadic endometrial tumors in the frequency of these mutations, the spectrum of p53 mutations was characteristically unique to the TAM-associated tumors. The median duration of TAM exposure was significantly longer in patients with p53 mutations than those without p53 mutations (62 vs. 30 months, p=0.028). Our observation suggests that prolonged TAM exposure may directly inactivate the p53 gene by acting as a mutagen in a significant fraction of TAM-associated endometrial carcinomas.