Elf-1 is a member of the Ets transcription factor family that regulates the genes involved in cellular growth and differentiation. Enhanced expression of Elf-1 has been reported in prostate cancer, breast cancer, and osteosarcoma. The aim of this study was to elucidate the involvement of Elf-1 in ovarian carcinogenesis. We analyzed serial frozen tissue sections from 26 patients with epithelial ovarian carcinoma and 10 patients with benign cystadenoma of ovary for Elf-1 protein expression, using fluorescent immunohistochemistry. We analyzed the relationship between the percentages of Elf-1-stained cells and patient characteristics, including histological classification, clinical stage, histological grade, clinical outcome, and survival rate. Elf-1 was weakly detected in some benign cystadenomas (0-5.5%). There was, however, abundant Elf-1 immunoreactivity in the nucleus of the ovarian carcinoma cells along with a little cytoplasmic staining. Scoring on the basis of the percentage of nuclear-positive cells indicated that nuclear Elf-1 expression was significantly associated with PCNA-labeling index (p=0.04), clinical stage (p<0.01), histological grade (p<0.01), and clinical outcome (p=0.02). However, there was no relationship between Elf-1 expression and histological classification. Survival data were available for all patients and demonstrated that Elf-1 expression was significantly associated with poor prognosis (p<0.01). Our results demonstrate that Elf-1 expression in ovarian carcinoma correlates with the malignant potential of this tumor.