To understand the pathogenesis of high-grade neuroendocrine carcinoma (HGNEC), we examined the histopathology and immunoreactivity against adenocarcinoma (AD), squamous cell carcinoma (SQ), and neuroendocrine markers in 34 cases with combined HGNEC. The 5 year overall survival rates of patients with combined small cell carcinoma (SCC) (n = 9) and combined large cell neuroendocrine carcinoma (LCNEC) (n = 25) were 33% and 75%, respectively (P = 0.011). Most of the patients were male (94%), smokers (94%), and had tumors located in the peripheral (94%) and upper lobe (65%) of the lung. Histopathologically, non-HGNEC components were predominantly ADs (65%) followed by SQs (26%). In combined HGNEC and AD, a lepidic AD component was found in 12 cases (48%). For the HGNEC components of combined HGNEC and AD, the incidence of positivity of thyroid transcription factor-1 (TTF-1) (8G7G3/1) and TTF1 (SPT24) were 64% and 91%, respectively. For HGNEC components of combined HGNEC and SQ, the incidence of positivity of 34βE12 and p63 were 22% and 11%, respectively. In conclusion, 48% of combined HGNEC and AD cases had a lepidic AD component, suggesting that HGNEC can develop in association with pre-existing AD. AD markers, but not SQ markers, were frequently retained through development of the HGNEC component.