Therapy for non–small cell lung carcinoma (NSCLC) is currently determined by histologic subtype and the presence or absence of actionable mutations. Accurate subclassification is therefore essential for appropriate selection of cases for molecular studies and guiding treatment. The gold standard for subclassification of NSCLC is identification of differentiating morphologic features in correlation with diagnostic immunohistochemistry (IHC) in cases of poorly differentiated carcinoma. Whereas Napsin A, TTF1, and p40 antibodies have been used individually for the subtyping of NSCLC, few studies have examined the 3 in cocktail form. Using a novel triple IHC antibody cocktail (TNP) composed of TTF1 (brown nuclear), Napsin A (red granular cytoplasmic), and p40 (red nuclear), a randomized, double-blinded subclassification was performed on a representative histologic section of 32 previously resected primary NSCLCs. TNP results were then compared with the gold-standard diagnosis. TNP accurately identified all (100%, 10/10) squamous cell carcinomas (SCCs) (p40+/TTF1−/Napsin A−) and 89% (16/18) of adenocarcinomas (ADCs) (p40−/TTF1+/Napsin A+). TNP was negative in 7 (20%) tumors (p40−/TTF1−/Napsin A−), including 2 mucinous ADCs. TNP showed no overlapping or discordant immunostaining. Using traditional IHC with p63, CK5/6, and TTF1, all TNP (−) cases remained unclassifiable. With the exception of mucinous ADC, which was TNP negative, all TNP cases correlated with gold-standard diagnosis; 78% of tumors were also definitively classified as either ADC or SCC and required only a single slide for classification.