The identification of markers that distinguish primary pulmonary adenocarcinomas from pulmonary adenocarcinomas secondary to the digestive tract would be clinically important. Villin, a specific marker in digestive-tract malignancies, was evaluated in 57 pulmonary adenocarcinomas, six samples of proximal bronchial tissue, and five metastatic pulmonary adenocarcinomas (three colon and two esophageal adenocarcinomas) by using immunohistochemical and molecular analyses. Villin was expressed in 31.6% (18 of 57) of the pulmonary adenocarcinomas and showed either a diffuse cytoplasmic pattern (10.5%) or a primary cytoplasmic pattern with minor brush-border staining (21.1%). However, none of those samples demonstrated the primary brush-border staining pattern that was characteristic of all five of the metastatic digestive-tract adenocarcinomas. There was a significant difference in the positive brush-border staining pattern between the primary and metastatic pulmonary adenocarcinomas (P < 0.002). Villin protein was expressed in bronchial epithelial cells, and villin mRNA was detected by reverse transcription-polymerase chain reaction. Northern analysis demonstrated 3.5- and 2.7-kb villin mRNAs in villin protein-positive tumors, but villin mRNA was not detected in non-tumorous lung tissue, indicating the transcriptional upregulation of villin in lung tumors. An additional smaller-sized mRNA (1.8 kb) was observed in six of 10 pulmonary adenocarcinomas and in the bronchoalveolar carcinoma cell line A549. Two small villin mRNAs were cloned from the cell line A549 and were found to represent an alternatively spliced (exon 8-exon 14) 1.85-kb mRNA and a 1.8-kb mRNA that was missing a portion of the 5' region (exon 1-exon 9) of the native villin mRNA. These studies demonstrated that the pattern of villin expression and the presence of altered villin mRNAs may be useful markers for pulmonary adenocarcinomas as well as provide support for the potential origin of villin-expressing tumors from bronchial epithelial cells.