Background: SCLC and L-LCNEC are aggressive neoplasms that are both associated with smoking history and are thought to overlap in clinical, histogenetic, and genomic features. We reviewed the genomic profiles of >1187 patients to assess the genomic similarities of these diseases. Methods: Comprehensive genomic profiling (CGP) of tumors from 300 L-LCNEC and 887 SCLC patients in the course of clinical care was performed to suggest pathways to benefit from therapy. Results: Commonly altered genes in both diseases included TP53, RB1, MYC/MYCL1, MLL2, LRP1B and PTEN; alterations in other genes occurred at somewhat differing frequencies (table). For both diseases, RB1 mutation significantly co-occurred with TP53 mutations (p<0.001), but occurred in a mutually exclusive fashion to STK11 and CDKN2A (p<0.001). RB1 was mutually exclusive with KRAS for L-LCNEC but not for SCLC. The interquartile range for SCLC and L-LCNEC TMB is 7.9 and 12.6 with the 75% quartile being 14.4 and 17.1 respectively. Cases of both diseases will be presented with radiographic response to genomically matched targeted therapy and immunotherapy, particularly in cases of high TMB. Conclusions: Given the similar overall genomic profiles and clinical behavior of a subset of these diseases, they could be conceived of as a single disease to be further classified by genomically defined classes such as SCLC-type (TP53/RB1mutated) and NSCLC-like (wild type for one or both). By analogy to NSLC and melanoma, benefit from immunotherapy appears most likely for only the upper quartile of cases in TMB.