The topic of pulmonary metastases has long been of high interest and ongoing controversy. There is a group of patients with pulmonary metastases who may benefit from curative resection. It remains unclear which among them will benefit from surgery in terms of survival. This work updates a previously proposed classification system for pulmonary metastases, similar in its essence to the tumor, nodes, metastasis (TNM) classification used for primary tumors and named pmTNM classification, where 'pm' stands for 'pulmonary metastasis'. The objective is to allow future studies to explore predictive and survival prognostic factors for pulmonary metastases and separate patients who will benefit from lung metastasectomy from those who will not. The secondary aim is to provide a classification system that will allow physicians, oncologists and surgeons to speak the same language in comparing their data and assessing the results of treatment of lung metastases.