Background. The nodal status remains the most important prognostic factor in breast cancer. While evaluation of the axillary lymph nodes remains a standard of practice, evaluation of the internal mammary lymph nodes is no longer routinely performed. In the era of extensive radical mastectomies, it was shown that up to 40% of breast cancer patients had nodal metastases in the internal mammary chain. This resulted in up to 10% of presumed "node-negative" patients actually being node-positive when the internal mammary nodes were examined. In the era of sentinel node biopsies, hot internal mammary nodes on lymphoscintigraphy are sometimes encountered and confusion exists regarding the appropriate approach to these nodes. New advances in endoscopic surgery have enabled a minimally invasive approach to the mediastinum. The aim of this study was to evaluate the feasibility of thoracoscopic internal mammary sentinel node biopsy in an animal model. Materials and methods. Five farm pigs were injected with isosulphan blue under the right upper nipple. After a sentinel node was identified, it was dissected thoracoscopically. Results. In all the animals, an internal mammary blue node was easily identified 1-5 min after the injection and dissected with thoracoscopic instruments without significant damage to other thoracic structures. The procedure length averaged 30 to 60 min. Conclusions. Thoracoscopic internal mammary sentinel node biopsies are feasible, short, easy to perform, minimally invasive, and well focused toward a sentinel node. Well-planned phase I studies should be initiated to further evaluate this new technique.