The possible role of a novel steroid receptor in ovarian malignancy was investigated. The evolutionarily conserved orphan steroid receptor COUP-TF (chicken ovalbumin upstream promoter transcription factor) was originally identified as a protein interacting with an upstream promoter element of the chicken ovalbumin gene. The human receptor protein was purified from a cervical cancer cell line. An immunocytochemical technique for the visualization of COUP-TF was developed using a specific polyclonal rabbit antibody. Four established ovarian cancer cell lines were evaluated. The patterns of COUP-TF expression were compared to the staining intensities of immunocytochemical assays for estrogen receptor (ER), androgen receptor (AR), aromatase, and HER2/neu. A comparison of the ovarian cancer cell lines showed differential expression of COUP-TF in the nucleus. The pattern of COUP-TF expression did not follow the profile of any of the other four variables. In agreement with transfection experiments showing reduction of activity of other steroid receptors by elevated COUP-TF levels, high COUP-TF expression correlated with low ER activity also in native ovarian cancer cells. These data represent the first reported evidence that COUP-TF-like proteins may play a role in the metabolism and possibly in the process of dedifferentiation of human ovarian cancer.