Carcinoma of the ovary is the leading cause of death from gynecological cancer in western countries. Ovarian carcinoma is commonly associated with the accumulation of fluid containing malignant cells in the peritoneal, and not infrequently in the pleural cavity. The differentiation of these cells from reactive mesothelial cells is at times difficult. In addition, tumor progression in ovarian carcinoma and the biological characteristics of carcinoma cells in effusions compared to their counterparts in solid tumors are poorly understood. This review details the current knowledge regarding diagnostic and biologic aspects of effusion cytology, with emphasis on ovarian carcinoma. Results from our first studies of effusions are subsequently presented. These attempt to address several issues. First, to improve the diagnostic ability to detect cancer cells in effusions using antibodies designed for the differentiation of epithelial cells from mesothelial cells. Secondly, to study genotypic and phenotypic differences between ovarian carcinoma cells in effusions, solid primary tumors and metastatic lesions, as well as to compare malignant cells in peritoneal and pleural effusions. These studies of carbohydrate antigens, E-cadherin complex and matrix metalloproteinases (MMP) attempted to evaluate whether ovarian carcinoma cells in effusions possess true metastatic properties, or are similar to the cells in primary tumors, thereby merely representing the result of a shedding process. Finally, the prognostic role of these molecules was studied in solid tumors from a patient cohort consisting of long- and short-term survivors, followed for up to 20 years. Figure 1 on http://www.esacp.org/acp/2001/23-3,4/davidson.htm.