Human breast cancers, especially estrogen receptor alpha (ER(alpha))-positive ones, often overexpress the oncoprotein MDM2 without mdm2 gene amplification. The mdm2 gene is transcribed into two different mRNAs, namely L-mdm2 and S-mdm2, which are generated from promoters P1 (constitutive) and P2 (regulated by tumor suppressor p53), respectively. To cast light on the mechanisms of MDM2 overexpression, we measured the expression levels of these mdm2 mRNAs using RT-PCR analysis in three human breast cancer cell lines and 15 breast cancer samples obtained from surgery. ER(alpha)-positive MCF-7 cells, which possess wild-type p53, displayed dominant expression of S-mdm2. In contrast, two other cell lines with mutant p53, T47-D (ER(alpha)-positive) and MDA-MB-231 (ER(alpha)-negative), showed almost equivalent expression of L-mdm2 and S-mdm2. Treatment of 17beta-estradiol (E2) significantly enhanced the expression of S-mdm2 but not that of L-mdm2 in MCF-7. Among 6 breast cancer samples regarded as ER(alpha)-positive with wild-type p53, 5 samples showed increased expression of S-mdm2. Expression of S-mdm2 was stimulated in 2 ER(alpha)-positive samples with mutant p53. In contrast, 4 of 5 samples which express mutant p53 without ER(alpha) showed poor expression of S-mdm2. There is a tendency that ER(alpha)-positive breast cancers with wild-type p53 preferably use P2 promoter for the expression of mdm2, possibly through E2-induced accumulation of p53. However, wild-type p53 and ER(alpha) are not necessarily enough for the utilization of S-mdm2. Tumors with mutant p53 also showed expression of S-mdm2 in some cases. These results strongly suggest that other factor(s) is also implicated in the promoter usage of mdm2 gene in human breast cancer tissues.