C19 steroids are converted to estrogens in a number of human tissues by the aromatase enzyme complex, which consists of aromatase cytochrome P450 (P450arom; product of the CYP19 gene) and NADPH-cytochrome P450 reductase. Aromatase activity has been previously demonstrated in endometrial tumors. In the present study, we investigated CYP19 gene expression and its regulation in endometrial tumor samples (n = 9). Using a specific method of competitive polymerase chain reaction after reverse transcription, varying levels of P450arom transcripts were detected in all endometrial adenocarcinomas (n = 8) and one mixed Müllerian tumor studied. No correlations were observed between P450arom transcript levels and histological type of the tumor, grade, myometrial invasion, stage of the disease, or patient age. We have recently demonstrated that the tissue-specific regulation of CYP19 gene transcription is in part the consequence of alternative promoter use. The use of each promoter gives rise to a P450arom transcript with a unique untranslated 5'-end. We analyzed the untranslated first exons in 5'-terminals of P450arom transcripts in endometrial adenocarcinomas using a specific reverse transcription-polymerase chain reaction/Southern hybridization method we recently developed. Our findings indicated the gonadal-type (promoter II) and one of the adipose stromal cell-type (I.3) promoters were primarily used for P450arom expression in adenocarcinomas. On the other hand, distribution of transcripts specific for I.3, I.4 (another adipose-type promoter), and promoter II in one mixed Müllerian tumor was uniform. Placental promoter (I.1)-specific P450arom transcripts were not detected in endometrial tumors. As P450arom transcripts were detected in all endometrial malignancies studied, whereas they were not demonstrable in the disease-free endometrium, activation or failure of inhibition of aromatase expression in these tumors may serve to promote neoplastic proliferation.