Invasion of renal cell carcinoma to veins is known to have a close relation with pulmonary metastasis. We speculated that a tumor thrombus would lodge in the pulmonary artery before establishment of a clinically apparent pulmonary metastasis. This may be particularly true in patients with renal vein or vena cava involvement of renal cell carcinoma. In this instance, it is crucial to know the clinical consequence of tumor thrombi in the pulmonary artery. Thus, we investigated these issues with the aid of lung-perfusion imaging in 22 renal cell carcinoma patients with and without vein involvement. The lung-perfusion imaging revealed positive in 8 of the patients examined prior to treatment. The incidence of positive finding was well correlated with an extensive vein invasion. Patients with positive imaging prior to treatment were associated with pulmonary metastasis at a higher rate than those with negative findings. These findings have indicated that vein invasion should be related with tumor thrombus formation in the pulmonary artery, and such a status in the lung would, in part, result in pulmonary metastasis. Two different clinical courses were found by an analysis of patients with the positive imaging prior to treatment. One is the clinical course in which positive findings were correlated with a newly developed metastasis in the lung. Thus, care should be taken in development of the metastasis during the follow-up when patients show the positive lung-perfusion imaging prior to treatment. On the other hand, we found two patients in whom the positive findings disappeared during the follow-up.