Using a cell line derived from human lung cancer (AOI), we successfully established human xenografts in KSN nude mice, which showed high incidence of multiple spontaneous metastases. The highest incidence of metastasis in untreated hosts was observed in the spleen followed by the lungs and lymph nodes. The rate of metastasis reached 100% in the mice bearing large sized tumors, when metastasis to any organ or tissue was counted. Experimental radiotherapy caused remarkable redistribution of metastatic foci among different organs. Lung metastasis tended to decrease, while metastasis to the liver and the kidney was increased after radiotherapy. Radiation-induced production of cytokines was speculated to be responsible for such an alteration of metastasis pattern.