A new nude-mouse metastasizing orthotopic transplant model of human small-cell carcinoma of the lung is described. Histologically-intact human small-cell lung tumors were transplanted to the left lung of nude mice via a thoracotomy procedure we have developed. The transplanted tumors grew extensively locally and metastasized to the opposite lung, lymph nodes and other clinically-relevant sites. The results described indicate the model developed could have clinical relevance and contrasts with models of small-cell carcinoma constructed with injections of cell suspensions which result in few or no metastases.