There is currently no suitable animal model of metastasis using cultured human gastrointestinal stromal tumor cells even though the molecular mechanisms of c-KIT-mediated progression and metastasis should be clarified. Ba/F3 murine lymphocyte cells transduced with mutant c-KIT have been utilized to analyze some molecular mechanisms related to a constitutively activated c-KIT signaling and to assess the efficacy of molecular-targeted inhibitors. Using this cellular system, we coincidentally discovered the development of axillary and inguinal lymph node swelling three weeks after subcutaneous injection of Ba/F3 cells with c-KIT mutation into nude mice. Mutation-specific PCR detected c-KIT mutation in the swollen lymph nodes but not in unmetastasized normal lymph nodes, indicating that the lymph nodes contain tumor cells which should come from a primary subcutaneous tumor. Microscopic observation revealed tumor cells infiltrating through lymphatic follicles with Ki-67-positive staining to distinguish them from lymphocytes. The significance of this model is helpful to understand the molecular mechanisms of c-KIT-mediated metastasis and is useful for assessments of molecular therapeutics and in vivo imaging.