The in situ formation of estradiol plays an important role in the development and biological behavior of human breast cancer Aromatase and 17 beta-hydroxysteroid dehydrogenase type 1 (17 beta-HSD type 1) are two principal enzymes involved in in situ estradiol production. We evaluated the expression of aromatase and 17 beta-HSD type 1 by immunohistochemistry in 41 cases of invasive breast carcinoma (19 lobular and 22 ductal). We then examined the correlation among the expression of these enzymes, estrogen (ER) and progesterone (PR) receptor status, Ki67 labeling index of carcinoma cells, age, and the clinical stage of the patients. Marked aromatase immunoreactivity was observed in stromal cells around carcinomatous glands in 32 of 41 cases (78%), and 17 beta-HSD type 1 immunoreactivity was detected in carcinoma cells in 23 of 41 cases (56%). There was a significant correlation observed between expression of 17 beta-HSD type 1 and aromatase in invasive lobular carcinoma (P = 0.0119), but not in invasive ductal carcinoma. There was an inverse correlation between aromatase and ER status in invasive ductal carcinoma (P = 0.0213), but not in invasive lobular carcinoma. No other correlations were observed among 17 beta-HSD type 1, aromatase, PR, ER, clinical stage, age, and Ki67 labeling indexes. Aromatase and 17 beta-HSD are not always expressed simultaneously in human breast carcinoma, but their simultaneous expression is more frequent in invasive lobular carcinoma than invasive ductal carcinoma. Consequently, different mechanisms may be involved in the regulation of expression of these two enzymes in human breast carcinoma.