In situ hybridisation was used to study the local expression of tumour necrosis factor (TNF) mRNA in human ovarian tumours. In 8 of 14 ovarian cancers studied, a minority of cells in the epithelial areas of the tumour contained TNF mRNA. In individual high-power fields as many as 8% of cells were positive for TNF mRNA. Immunohistochemical studies on sequential sections and the morphology of the positive cells led to the conclusion that the ovarian tumour cells were transcribing the TNF gene. There was immunohistochemical evidence of the production of TNF protein by the tumour cells and TNF protein in a tumour lysate. The production of TNF by human ovarian cancer cells may influence the biology of the tumour, contribute to neoplastic progression and alter the response to therapy.