In order to examine selective lung metastasis in lung adenocarcinoma, 7 patients who had died due to metastasis exclusively to the lung were analyzed. Because all of the tumors had been well differentiated adenocarcinoma, 21 cases of well differentiated adenocarcinoma which had metastasized to both the lung and other distant organs were selected as controls. Histologically, 6 of the 7 study cases were bronchiolo-alveolar carcinoma (BAC) of variable cytological type or papillary adenocarcinoma with a thin fibrovascular stroma without apparent destruction of the alveolar wall. There was no difference in clinical findings or nuclear DNA content (NDC) between the BAC and papillary adenocarcinoma with thin fibrovascular stroma. The fibrovascular stroma in the study cases was significantly thinner than that in the control cases (P less than 0.01). Cytofluorometrically, the study cases had a significantly lower and less abnormal NDC than the control cases in terms of the mean NDC and DNA histogram patterns (P less than 0.01 and P less than 0.05, respectively). On the basis of these findings, we conclude that 1) BAC can be classified within the same spectrum as well differentiated papillary adenocarcinoma, and 2) selective lung metastasis occurs in well differentiated adenocarcinomas showing mild NDC abnormalities and having a thin fibrovascular stroma without apparent destruction of the alveolar wall.