Abnormalities of p53 gene are common in lung cancers and are associated with immunologically detectable p53 protein. p53 immunoreactivity is uncommon in normal cells but is frequently seen in neoplasia. Therefore, assessment of p53 expression may assist in the cytological diagnosis of malignancy. The usefulness of p53 immunostaining as a marker of malignancy in the cytological analysis of bronchial brush specimens from the patients with lung cancers was investigated in this study. A total of 71 bronchial brush samples submitted for cytologic diagnosis were immunostained with D07, a monoclonal antibody to recombinant p53 protein. Resultant p53 data were correlated with cytologic diagnosis and clinical information. Of the 17 smears with a benign cytodiagnosis, all were p53 negative. Of the 40 cases with a malignant cytodiagnosis (histologically confirmed), 35 were p53 positive and 5 were negative. Of the 14 cases that were cytologically suspicious but nondiagnostic for malignancy, 11 were p53 positive, 9 of which were subsequently proved to be malignant by histologic examination, and the remaining 2 cases were tuberculosis clinically. Forty four of 51 histologically confirmed lung carcinomas were p53 positive, including 25 of 28 squamous cell carcinomas, 13 of 17 small cell carcinomas, 3 of 3 adenocarcinomas, and 3 of 3 large cell undifferentiated carcinomas. These results suggest that p53 immunostaining could be of value as a marker of malignancy in the cytologic examination of bronchial brush specimens. Furthermore, we have shown the possible clinical utility of p53 immunostaining in cytopathological diagnosis, that is, as a valuable adjunct to morphological assessment in the analysis of cytopathologically suspicious cases.