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The World Health Organization classification of lung tumors recognizes 4 histologic subtypes of pulmonary neuroendocrine carcinomas (NECs), which include typical carcinoids (TCs), atypical carcinoids (ACs), small cell carcinomas (SCCs), and large cell NECs (LCNECs). These tumors can be misclassified owing to morphologic parallels, indicating the necessity for adjunctive tests for correct classification. We evaluated immunohistochemical expression of PAX-5 in histologic and fine-needle aspiration (FNA) specimens of pulmonary NECs. Staining was stratified by intensity (0 to 3+) and percentage of cells stained as focal (<10%) or diffuse (≥10%). PAX-5 expression was present in 29/37 (78%) of high-grade NECs (22/26 SCCs, 1/2 LCNECs, and 6/9 combined tumors) and none of 51 TCs and ACs; FNA specimens showed concordant staining. This study confirmed that PAX-5 is a useful marker in FNA and surgical specimens for the discrimination of low- to intermediate-grade NECs from high-grade NECs with 100% specificity and 79% sensitivity in surgical specimens.