The heterogeneity of tumor cells is frequently observed in lung cancer, but the clonality of these cells has not yet been established. The distinct components of 12 lung adenosquamous carcinomas were compared by genetic alterations of p53 and K-ras, chromosomal abnormalities at 9p21 and 9q31-32, and immunohistochemical reactions. The immunoreactivity of p53 was consistent in both adenocarcinomatous and squamous cell carcinomatous components as well as in the transitional areas, retaining the morphological characteristics of the distinct components. The same p53 mutation was found in both components of each tumor with p53 overexpression. No K-ras mutations were detected in any of the tumors examined. Three of the four tumors with chromosomal abnormalities detected, one at 9p21 and two at 9q31-32, had coincident abnormalities between the distinct components, whereas one tumor deleted homozygously at 9p21 (D9S259) in the adenocarcinomatous component with loss of heterozygosity in the other component. The expression of squamous cell carcinoma-related antigen in adenocarcinomatous components was significantly higher than that of lung adenocarcinomas (57 +/- 5.8% vs. 1.0 +/- 0.5%, P < 0.0001), whereas Mucin 1 expression is less in these components (9.0 +/- 4.9% vs. 55 +/- 8.2%, P = 0.003). These results suggest monoclonal transition from squamous cell carcinoma to adenocarcinoma in lung adenosquamous carcinoma.