We demonstrate here the visualization of human lung cancer metastasis live and in process in nude mice by green fluorescent protein (GFP) expression. The human lung adenocarcinoma cell line Anip 973 stably transfected with the humanized GFP-S65T cDNA was selected for very bright green fluorescence. GFP-transfected lung cancer cells were initially inoculated subcutaneously in nude mice. Five weeks after transplantation, the resulting tumor had reached over 1 cm in diameter and had very bright GFP fluorescence. Fragments of subcutaneous tumor were implanted onto the visceral pleura of the left lung of nude mice by surgical orthotopic implantation (SOI) of histologically-intact tissue via transverse thoracotomy. The ipsilateral resulting tumor was highly fluorescent due to GFP expression. GFP expression allowed the visualization of the advancing margin of the ipsilateral tumor into the fresh normal lung tissue. Lymphogenous and direct-seeding metastases in the pulmonary hilum, cervical lymph nodes, the mediastinum and contralateral pleural cavity and contralateral lung in the SOI-treated mice were brightly visualized by GFP expression in fresh tissue. GFP-transfected and untransfected tumor had similar metastatic characteristics suggesting that GFP expression had no effect on metastasis itself. The results with the GFP-transfected tumor cells, combined with the use of SOI, demonstrate a fundamental advance in the visualization and study of lung cancer metastasis in process.