We have constructed a metastatic model of human non-small cell lung cancer (NSCLC) by injection of NSCLC cell lines directly into the left pleural cavity of BALB/c nude mice. All of seven NSCLC cell lines, which are tumorigenic after subcutaneous injection, were successfully transplanted in the pleural cavity, while only three of the seven cell lines produced lung metastatic colonies after intravenous injection. Tumors grew extensively in the pleural cavity and infiltrated into the lung parenchyma directly. Furthermore, tumors metastasized to the mediastinum and contralateral pleural cavity through lymphatic routes. Since this model is easy to perform and the result is reproducible, it would be useful for studies on the biological behavior and treatment of human NSCLC in vivo.