Aromatase (P450AROM) is the enzyme complex with converts testosterone to estradiol and androstendione to estrone. This enzyme was detected in various normal tissues and uterine pathology such as uterine myoma, endometrial cancer and endometriosis. The aim of the study was to estimate expression of P450AROM messenger ribonucleic acid (mRNA) in normal, hyperplastic and malignant endometrium, and the ability to convert androstenedione to estrone by endometrial cancer tissue. Normal endometrium was obtained from 16 (12 proliferative phase, 4 secretory phase) regularly cycling women after hysterectomy for myomas, hyperplastic endometrium (n = 5) and endometrial cancer (n = 5) from postmenopausal women. The ability to convert androstenedione to estrone was estimated in 16 cases of endometrial cancer in postmenopausal women. P450AROM mRNA was measured by a quantitative assay based on reverse transcribing the mRNA into cDNA with reverse transcriptase (RT) then amplification of the cDNA using the polymerase chain reaction (PCR). The mean (+/- SEM) expression of aromatase gene in proliferative endometrium was 84.4 +/- 14.0 pg mRNA/microgram DNA and in secretory endometrium 200.3 +/- 87.8 pg mRNA/microgram DNA. The mean (+/- SEM) P450AROM mRNA expression in endometrial hyperplasia was 92.9 +/- 17.8 pg mRNA/microgram DNA, in endometrial cancer was 14.3 +/- 7.7 pg mRNA/microgram DNA. Androstenedione to estrone conversion in endometrial cancer tissue culture was 252.5 +/- 91 fmol/g tissue/h. Our data confirm that human normal, hyperplastic and malignant endometrium do express P450AROM mRNA and that aromatase activity is present in endometrial cancer tissue.