Several investigators have noted that hormone-dependent development of endometriosis implants lags behind that of simultaneously analysed eutopic endometrium. With the recent discovery of the oestrogen receptor-beta (ER-beta) isoform, the aim of this study was to investigate whether differences in the expression of ER-alpha and ER-beta might explain this observation. mRNA transcripts from endometrial stromal cells isolated from normal endometrium (NE) and from endometriomas (EI) were analysed using a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) technique. RT-PCR and Southern blot analyses of the two ER isoforms indicated that NE and EI stromal cells predominantly express ER-alpha mRNA, however the relative concentrations of ER isoform mRNA transcripts differed between the two cell types. Steady-state ER-alpha:ER-beta mRNA ratios were 15.5 +/- 2.8 and 5.2 +/- 0.9 respectively for NE and EI cells (P = 0.02). NE and EI stromal cells expressed ER proteins with similar Kd ( approximately 0.9 nM) and densities ( approximately 24 500 binding sites/cell) respectively. Functional ER expression was indicated by an increase in progesterone receptor concentrations of approximately 60% (P = 0.03) after incubation with 10 nM oestradiol. We postulate that differential transcript processing, ligand specificity and biological actions of the ER-alpha and -beta isoforms may influence differential growth responses in normal and ectopic endometrium.