The possibility that the female sex steroid progesterone plays a role in meningioma proliferation has been suggested by a number of investigators; and it had been shown that many meningiomas have high affinity progesterone binding sites. The aim of this study was to examine the expression of progesterone receptor mRNA and correlate it with the nuclear localization of progesterone receptor by immunocytochemistry in a large number of meningiomas. Thirty-three meningiomas were examined for the presence of measurable amounts of progesterone receptor mRNA by Northern blot analysis and 11 of these were analyzed for receptor protein by immunohistochemistry. These were compared with normal arachnoid and an arachnoid cell line. Sixty-four % of the meningiomas expressed progesterone receptor mRNA. This occurred in a similar pattern to what has previously been shown for T-47 D cells, a breast carcinoma cell line with 11.4-, 6.1-, 5.2-, 4.5-, 3.2-, and 2.5-kilobase mRNA species. This suggests that progesterone receptor expression is not tumor specific. There was a marked predominance of women among those patients whose tumors expressed progesterone receptor; 81% were female and 19% were male. The immunohistochemistry data correlate well with the Northern blot analysis. The staining was clearly nuclear, suggesting that the receptor is in a location to be activated. These data suggest that progesterone receptor mRNA and protein is expressed in meningiomas and support the concept that progesterone may play an important role in meningioma growth.