Previous studies have localized basic fibroblast growth factor (bFGF) and its mRNA in normal and neoplastic endometrium but the expression of bGFG mRNA in endometriosis cells is virtually unknown. Our purpose was to investigate the presence of bFGF and its receptor mRNAs in highly purified primary cultures of stromal cells isolated from six eutopic endometrial samples obtained from patients without evidence of endometriosis and five endometriosis cyst linings. Using reverse transcriptase-polymerase chain reaction (RT-PCR), single major DNA bands of the expected sizes for bFGF and its receptor (354 and 661 bp, respectively) were detected in both endometrial and endometriosis samples. A competitive RT-PCR, that uses coprimer extension and amplification of a bovine RNA as an internal standard, was developed for semiquantitative estimation of bFGF gene expression. The target to standard mean ratios +/- SEM in six normal endometrial samples and in five endometriosis cultures were 26.7 +/- 10.7 and 9.2 +/- 3.0, respectively. Furthermore, when data were analyzed according to the cyst diameter, levels of bFGF mRNA resulted statistically higher (P < 0.05) in bigger cysts when compared to those detected in smaller ones (16 +/- 2.7 and 4.7 +/- 1.8, respectively). These results demonstrate that the genes coding for bFGF and its receptor are expressed in endometriosis cells, but levels of bFGF mRNA are generally lower than those detected in their eutopic counterpart. Moreover, they indicate that endometriosis cells derived from large cysts have increased bFGF mRNA levels. Thus, bFGF could be one of the factors responsible for a more or less active behavior of the endometnotic lesion.