The ras-encoded p21 protein expression was investigated in 18 normal endometrial tissues and in 37 human primary endometrial carcinomas by Western blotting analysis. Scattered p21 levels were found in normal specimens (mean = 1.29 OD; median = 1.10 OD; range = 0.33-2.65). The p21 levels were significantly higher in secretory (mean = 1.99 OD; median = 2.16 OD; range = 0.71-2.65) than in proliferative (mean = 0.97 OD; median = 1.07 OD; range 0.38-1.73) endometrium (P = 0.009) and higher in primary endometrial carcinomas (mean = 2.05 OD; median = 2.04 OD; range 0.21-4.36) than in normal proliferative tissues (P = 0.004). Immunohistochemical analysis showed that most of the tumor cells expressed p21 oncoprotein while the stromal component was unreactive. No correlation between p21 expression and histopathological characteristics of the tumors was observed. Moreover, estrogen receptor (ER)-positive tumors expressed higher p21 levels than did ER-negative tumors (77% vs 33%; P = 0.009). A similar trend, although not statistically significant, was found between p21 values and progesterone receptor expression (74% vs 44%; P = 0.060). On the other side, p21 levels were unrelated to epidermal growth factor receptor levels. Further studies should verify the possible significance of p21 expression in the prognostic characterization of patients with endometrial cancer.