The cytomorphologic observations of peritoneal fluid from seven patients with ovarian serous adenocarcinomas are presented. Some different cytologic findings, including smaller cell and nuclear size, larger relative nuclear area, fewer cells with large discrete cytoplasmic vacuoles and macronucleoli, more frequent multinucleolation, and larger closely packed clusters of cells, were shown in serous adenocarcinoma of the ovary as compared with those in other types of ovarian malignant neoplasms. Although the materials of nonserous tumors are too limited for a valid comparative study, prediction of histologic types by the peritoneal fluid cytology will be possible if the close cytomorphologic studies are performed.