To determine the clinical significance of intraperitoneal cancer cells, comparative cytologic studies on ovarian cancer cells in peritoneal washings and ascites were performed. The materials obtained from 6 patients with ovarian cancer were examined by light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the results achieved were as follows: Ovarian cancer cells were recognized in both peritoneal washings and peritoneal effusion. It was easier to get a diagnosis from the cytologic features of the peritoneal washings than those of the effusion, because of the fewer contaminants such as fibrin, red blood cells and other degenerated tissues. Despite the presence of clustered cells in the washings, the nuclei and cytoplasmas were clearly seen. On SEM observation of the clustered cells from the effusion, red blood cells and fibrin were seen to adhere to the surface of cancer cells. Ultrastructural observation of cancer cells by TEM confirmed the fact that the activity of the cells was maintained even in ascites.