&NA;Cytologic diagnosis of adenocarcinoma metastatic to the peritoneal surface is often hampered by the difficulty of differentiating between reactive mesothelial cells and macrophages from malignant epithelial cells by light microscopy. Within vitrotissue culture technics, the fibroblastic dedifferentiation of mesothelial cells facilitates the recognition of residual neoplastic epithelial cells. Ascitic fluid from 18 patients who ultimately had histologic documentation of intraabdominal adenocarcinomatosis was studied by tissue culture technics. In 15 of the 18 patients, small islets of replicating tumor cells could be identified after the mesothelial cells had dedifferentiated into fibroblasts. With one exception the average time between collection of the specimen and recognition of adenocarcinoma was 6 days. In 14 cases, concomitant cytologic analysis was available. In 10 cases, a definitive diagnosis of malignancy was made by cytologic analysis.