INTRODUCTION: Patients with lung cancer who harbor multiple pulmonary sites of disease have been challenging to classify; a subcommittee of the IASLC Staging and Prognostic Factors Committee was charged with developing proposals for the 8(th) edition TNM classification to address this issue. METHODS: A systematic literature review and IASLC database analysis was performed to develop proposals for revision in an iterative process involving multispecialty international input and review. RESULTS: Details of the evidence base are summarized in other papers. Four patterns of disease are recognized; the clinical presentation, pathologic correlates and biologic behavior of these suggest specific applications of TNM classification rules. First, it is proposed that second primary lung cancers be designated with a T, N and M category for each tumor. Second, tumors with a separate tumor nodule of the same histologic type (either suspected or proven) should be classified according to the location of the separate nodule relative to the index tumor - T3 for a same-lobe, T4 for a same-side (different lobe), and M1a for an other-side location - with a single N and M category. Third, multiple tumors with prominent ground glass (imaging) or lepidic (histology) features should be designated by the T category of the highest T lesion, the number or "m" in parentheses (#/m) to indicate the multiplicity, and a collective N and M category for all. Finally, diffuse pneumonic-type lung cancers are proposed to be designated by size (or T3) if in one lobe, T4 if involving multiple same-side lobes, and M1a if involving both lungs with a single N and M category for all areas of involvement. CONCLUSION: We propose to tailor TNM classification of multiple pulmonary sites of lung cancer to reflect unique aspects of 4 different patterns of presentation. We hope this will lead to more consistent classification, clarity in communication, and facilitate further research in the nature and optimal treatment of these entities.