Implantation of malignant cells along the needle tract is an extremely rare but potential complication following percutaneous needle aspiration biopsy of malignant lesions. Percutaneous fine-needle aspiration biopsy (FNAB) has recently received more attention for cytologic diagnosis of bronchogenic carcinoma because of its high diagnostic yield, simplicity, and low morbidity. On the other hand, dissemination of cancer cells by needle aspiration biopsy can change a potentially resectable localized lung cancer to an unresectable one. We report two cases: one patient underwent FNAB of a metastatic left adrenal mass that seeded a paraspinal muscle implantation of malignant cells that subsequently developed a tumor mass, and the second patient had tumor cell implantation in the chest wall after FNAB of a pleural-based adenocarcinoma of the lung. The theoretical and practical importance of tumor cell spread along the needle tract is discussed. Because of its rare incidence, however, this complication should not affect the use of needle aspiration biopsy in bronchogenic carcinoma, although care should be undertaken during the procedure.