OBJECTIVE: To investigate the role of human epididymis protein 4 (HE4) in the diagnosis and subclassification of lung cancer and to compare it to TTF-1 and Napsin-A. STUDY DESIGN: A total of 141 patients with a diagnosis of lung carcinoma were enrolled in the study. The tumors were subclassified as small cell lung carcinoma (SCLC), squamous cell carcinoma (SqCC), and adenocarcinoma (AC) based on histology and immunohistochemistry for CD56, TTF-1, Napsin-A, p63, HMWK, and/or CK5/6. The results were compared to immunohistochemical expression of HE4. RESULTS: Using routine immunohistochemistry, 54 SCLCs, 42 SqCCs, and 45 ACs were identified. The majority of the SCLC and SqCC cases were devoid of HE4 expression (90.9% and 89.7%, respectively). On the other hand, the majority of the AC cases (78.8%) expressed HE4 protein immunohistochemically, and the difference was statistically significant (p=0.0001). In the AC group, TTF-1 and Napsin-A expressions were 70% and 68.75%, respectively. Interestingly, mucinpositive tumors and ACs with a lepidic growth pattern also expressed HE4 protein. CONCLUSION: This study shows that the immunohistochemical application of HE4 is easy and it is useful in distinguishing adenocarcinomas (particularly mucin-positive AC as well as lepidic pattern AC) from SCLCs and SqCCs of the lung. Its sensitivity has been superior as compared to TTF-1 and Napsin-A. © Science Printers and Publishers, Inc.