Mutant form of the p53 gene product is abnormally accumulated in the nuclei of the tumor cells due to prolonged half life, and readily detected by immunohistochemical methods. To determine the positivity rate of p53 in body cavity fluid according the primary site and histological types of tumors and the utility of p53 immunostaining as an adjunct in the diagnosis of malignancy, we reviewed 69 effusions, including pleural effusion, ascitic fluid, and pericardial fluid, that were diagnosed as overt malignancy and 21 effusions of suspicious malignancy, immunohistochemistry was performed on paraffin-embedded cell blocks using a monoclonal antibody to p53 supressor gene product(Clone DO7) and a standard avidin-biotin complex technique with a citrate buffer antigen retrieval solution. The results were as follows; of the 46 pleural effusions with overt malignancy, 22 were immunopositive for p53 protein; of the 21 ascitic fluids with overt malignancy, 5 were positive for p53. Positivity rates according to the primary sites of tumors were 18 of 34(52.9%), 8 of 21(38.1%), 1 of 9(11.1%) cases of the tumors of the lung, GI tract, and ovary, respectively. According to the histologic types of lung cancer, 11 cases(61.6%) were positive out of 18 adenocarcinomas, 2 of 5 large cell undifferentiated carcinomas, and 1 of 2 small cell undifferentiated carcinomas. Of 21 cases of suspicious malignancy, 6 were positive for p53 and all of them(6/6) were confirmed as adenocarcinoma of the lung or GI tract. These findings indicate that p53 immunostaining using paraffin embedded cell block is useful diagnostic and prognostic marker in body fluid cytology although negative immunostaining does not exclude malignancy.