Endometriosis is an estrogen-dependent disease of women of reproductive age. Recent studies demonstrate that endometriosis per se express high levels of estrogen synthetase (aromatase P450). The resulting estrogen synthesized in situ may play a role in the development and exacerbation of the disease. For ovarian endometrioma, previous studies have been conducted ex vivo using cells obtained from endometrioma and have demonstrated that steroidogenic factor-1 is involved in the expression of aromatase. The aim of the present study was to provide in vivo evidence that steroidogenic factor-1 plays an important role in the regulation and overexpression of aromatase P450 in situ. First, promoter use of aromatase P450 in endometrioma tissue was determined using quantitative methods. Ovarian endometrioma tissue was chopped into small pieces, and two exon 1-specific transcripts of aromatase P450 (PII-specific and I.4-specific transcripts) were quantified using competitive RT-PCR. PII-specific transcript was more abundant than the I.4-specific transcript in 13 of the 15 endometriomas and less abundant in the remaining two. Spatial distribution of aromatase P450 transcripts in these endometrioma tissues revealed heterogeneous expression in the cyst wall, demonstrating wide variability even in the same endometrioma. Two possible regulators of aromatase expression (steroidogenic factor-1 and IL-1 beta) were then measured in all endometrioma samples and the correlation between aromatase P450 transcripts and these possible regulators in the endometrioma samples were tested using Spearman's rank order correlation test. Levels of steroidogenic factor-1 transcript were found to correlate closely with levels of PII-specific transcript in eight of nine endometriomas examined. On the other hand, the level of IL-1 beta weakly correlated with I.4-specific transcripts in three of the nine endometriomas. We next histologically examined samples of four endometriomas in which complete sets of tissue samples corresponded to the RNA samples. We could not identify any specific pathology to explain the heterogeneous expression of PII-specific transcripts of aromatase P450, although the number of CD-68 positive macrophages in the tissue sections weakly correlated with the level of I.4-specific transcript in two of four endometriomas. These results provide strong evidence that promoter II is the predominant promoter of aromatase P450 in endometrioma tissues in vivo and that steroidogenic factor-1 in situ is a major determinant of aromatase P450 overexpression in endometrioma tissues in vivo.