Cytodiagnosis of pleural and ascitic fluid is a commonly performed laboratory examination. Especially, positivity for malignant cells in effusion cytology is very effective and also presents the first sign of malignancy in unknown primary site of the tumor. We examined each 34 cases of pleural and ascitic fluid cytologic specimen diagnosed as metastatic tumor, which was selected among 964 pleural fluid cytology cases and 662 ascitic fluid cytology cases from September 1989 to June 1995. Among the pleural fluid cytology specimens examined, 34 specimens were positive in 27 patients. The lung was the most frequent primary site(44%), followed by the stomach (12%), lymphoreticular neoplasm(12%), pancreas(3%) and colon(3%). And the cases of unknown primary site with positive pleural biopsy alone were 24%. Among trio ascitic fluid cytology specimens examined, 34 specimens were positive in 29 patients. The most common primary neoplasms. were carcinomas of ovary(32%), stomach(22%), colon(6%), breast(3%), pancreas(3%), and lung(3%) and lymphoreticular neoplasms(3%) The metastatic tumor was predominantly adenocarcinoma type in both pleural(82%) and ascitic(91%) fluid. The study of metastatic adeno- carcinoma in effusion from lung, ovary, and stomach was undertaken to find distinctive features for the identification of the primary site. The smears of metastatic pulmonary adenocarcinoma had a tendency to show high grade pleomorphism and many large tight cell clusters, whereas that of the ovarian adenocarcinoma showed low grade pleomorphism with abundant intracytoplasmic vacuoles in relatively clear background. That of the stomach revealed the intermediate features.