The p53 gene is well known as a tumour suppressor gene. In addition, the mutated p53 gene is detected in a variety of human cancers including lung cancer, and is considered as an oncogene. Lung cancer is also frequently associated with interstitial lung diseases. Therefore, it may be possible to hypothesize that there might be some abnormality of p53 gene in interstitial lung diseases. This work examined the relationship between the p53 protein and gene in lung tissues of 28 patients with interstitial lung diseases. Among 28 patients, 13 cases were pathologically diagnosed to have usual interstitial pneumonia (UIP), 12 cases were diagnosed as having collagen vascular lung diseases, and three cases were diagnosed to have a non-specific interstitial pneumonia. Twenty-three tissue samples were obtained by open lung biopsy and five samples were taken by autopsy. Paraffin-embedded tissues were treated by microwave, and stained with an anti-p53 antibody (DO7) by the Avidin-Biotin-Peroxidase (ABC) method. In selected patients, mutations in exons 5-8 of the p53 gene were also examined by single-strand conformation polymorphism (SSCP) analysis and DNA sequence. In addition, the presence of anti-p53 antibodies in patients' sera was screened for by ELISA. Fifteen samples (53.6%) revealed overexpression of the p53 protein in the nuclei of alveolar epithelial cells. However, SSCP or sequence analysis, which was performed in 13 tissues, showed no mutations in exons 5-8 of the p53 gene. In conclusion, p53 proteins were overexpressed in interstitial lung diseases, and the expressed p53 protein was considered to be wild-type. This wild-type p53 protein may play a role in blocking the transformation of proliferative epithelial cells.