The conversion of C19 steroids to estrogens occurs in a number of tissues and is catalyzed by a specific form of cytochrome P450, namely aromatase cytochrome P450 (P450arom). Previously, conversion of radiolabeled androstenedione to estrone has been demonstrated in uterine leiomyomas. By use of reverse transcription followed by polymerase chain reaction amplification of total RNA together with a rat cRNA as an internal standard, we detected and quantified P450arom transcripts in total RNA isolated from 32 of the 35 leiomyoma (91%) and from 18 of the 24 adjacent myometrial (75%) tissue samples from 26 women. P450arom transcripts were not detectable in myometrial tissues from disease-free uteri (n = 8). P450arom transcript levels in leiomyomas were similar to those in adipose tissue (normalized to total RNA) and were 1.5- to 25-fold higher than those in adjacent myometrial tissues. We did not find any correlation between P450arom transcript levels and leiomyoma size, histopathology, uterine weight, or patient age. In leiomyoma smooth muscle cells in culture (n = 4) and tissue explants (n = 4), aromatase activity was stimulated by dibutyryl cAMP, and this effect was potentiated by a phorbol ester. These increases in aromatase expression were accompanied by comparable increases in the levels of translatable P450arom mRNA. Treatment with dexamethasone or platelet-derived growth factor did not stimulate aromatase expression. Consistently higher levels of aromatase activity and P450arom transcripts were found in the leiomyoma tissues than in smooth muscle cells in culture (2- to 20-fold). Reverse transcription-polymerase chain reaction analysis of untranslated 5'-termini of mRNA species in leiomyomas revealed the use of primarily promoter II (the ovarian-type promoter) for CYP19 gene transcription. Leiomyomas also contain some transcripts with untranslated exon I.4 (previously found in adipose stromal cells and skin fibroblasts). Placental-type promoter-specific 5'-ends were not present in leiomyomas. We conclude that aromatase expression in leiomyomas is regulated by the rate of CYP19 gene transcription, which is, in turn, regulated by the use of tissue-specific promoters. These findings are consistent with the hypothesis that localized estrogen biosynthesis may be of pathological significance in the promotion of leiomyoma growth.