Metastatic tumors in the brain, liver, and regional lymph nodes (20 cases each) from patients with adenocarcinoma of the lung were examined by cytofluorometric analysis, and compared with the respective primary lung tumors. The nuclear DNA content of tumor cells was significantly increased in metastatic tumors in the brain and liver compared with the primary (P less than 0.01). However, the DNA content of metastatic tumors in regional lymph nodes was almost identical to that of the primary tumor in many instances. From the viewpoint of the nuclear DNA content of lung adenocarcinoma, blood-borne tumor cells in the brain and liver were considered likely to constitute a discrete tumor cell subpopulation, i.e., probably a more malignant one, different from the major subpopulation in the primary tumor, whereas lymphatic metastases in regional lymph nodes were similar to the primary. The subpopulation with an increased DNA content in hematogenous metastases were thought to have originated from a minor subpopulation in the primary tumor or to have developed at the metastatic site.