There have been a number of indications that sex hormones can affect the rate of growth of meningiomas during pregnancy. The presence of oestrogen or progesterone receptors in meningiomas and the influence of sex hormones upon cell cultures derived from human meningiomas have been extensively investigated. The results have been controversial, with most of the discussion centring upon the presence and possible role of oestrogen receptors. The aim of the present study was to assess oestrogen receptors in human meningiomas with 16alpha[l8F]fluoro-17beta-oestradiol ([18F]FES) and positron emission tomography (PET). With this purpose in mind, we measured the regional brain uptake of [18F]FES in six patients with a neuroradiological and histological diagnosis of meningioma, comparing the in vivo PET data with the immunohistological analysis of oestrogen receptors performed on formalin-fixed, paraffin-embedded tissue obtained at surgery. Two analyses were used for the in vivo measurement of [18F]FES binding to oestrogen receptors: the ratio of tumour activity to that of normal tissue (T/NT), calculated 90 min after tracer injection, and the ratio between the equilibrium distribution volume (DV) in normal and pathological tissues, calculated by means of a graphical kinetic analysis. PET studies demonstrated a marked uptake of [18F]FES by the tumour in four of the six patients. Immunohistochemical assay using a manual staining method capable of detecting oestrogen receptors at a level of > 10 pmol mg(-1) of protein detected only sparse immunostaining in one of the six meningiomas. Distinct albeit weak immunostaining was demonstrated in five of the six meningiomas when the sensitivity of the immunohistochemical assay was increased to < 10 pmol mg(-1) of protein by use of an automated staining method (Bioteck 1000). Comparison of the in vivo and immunohistochemical results showed a correlation in five of the six patients, thus indicating the high sensitivity of [18F]FES for the in vivo evaluation of oestrogen receptor expression.