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BACKGROUNDSmall-cell lung cancer (SCLC) is an aggressive malignancy with a poor prognosis and marked transcriptional heterogeneity that may drive distinct therapeutic vulnerabilities. Clinical translation of molecular subtyping has been limited by restricted access to tumor biopsies, particularly at relapse.METHODSWe applied chromatin immunoprecipitation of cell-free nucleosomes carrying active histone modifications followed by sequencing (cfChIP-seq) to 441 plasma samples from individuals with advanced SCLC, other neuroendocrine carcinomas, or non-SCLC cancers, as well as from healthy controls. Plasma cfChIP-seq profiles were integrated with matched tumor transcriptomes from 73 samples, including 41 time-matched pairs.RESULTScfChIP-seq captured the epigenetic and transcriptional landscape of tumor-derived cell-free DNA (cfDNA), including SCLC tissue- and cell-of-origin signatures. A quantitative cfChIP-seq-derived SCLC score tracked radiographic tumor burden and was associated with prognosis. Signals at promoters of lineage-defining transcription factor genes, including ASCL1, NEUROD1, POU2F3, and ATOH1, correlated strongly with matched tumor RNA expression and supported noninvasive inference of SCLC transcriptional subtypes directly from plasma.CONCLUSIONPlasma cfChIP-seq provides a practical liquid biopsy platform for real-time assessment of tumor burden, tumor state, and molecular subtype in SCLC. These findings support further development of cfChIP-seq for precision monitoring and subtype-informed therapeutic stratification in SCLC.TRIAL REGISTRATIONClinicalTrials.gov NCT02484404, NCT02487095, NCT02769962, NCT03554473, NCT03896503, and NCT02146170.FUNDINGCenter for Cancer Research; Intramural Program of the NCI (ZIA BC 011793); European Research Council (ERC) (Adg no. 101019560 "cfChIP").