BACKGROUND: MUC4 is a transmembrane glycoprotein that plays a role in the cell growth signaling pathway and has been studied in various organ malignancies. This study aimed to analyze MUC4 expression in resected lung adenocarcinomas (ADCs) to define the clinicopathologic characteristics of MUC4-positive cancers. PATIENTS AND METHODS: Immunohistochemical MUC4 analysis was performed using tissue microarray slides containing 338 lung ADCs. Associations between MUC4 expression and the following clinicopathologic parameters were evaluated: sex; age; smoking status; tumor stage; tumor grade; lymphovascular invasion; pleural invasion; TTF-1 and HNF4α expression; EGFR, KRAS, BRAF, and HER2 mutation status; and ALK and ROS1 fusion status. RESULTS: Ninety-four tumors (27.8%) were MUC4 positive. Most patients with MUC4-positive tumors were male (P < .001) and smokers (P = .006). Moreover, MUC4 expression was significantly associated with solid ADCs (P < .001) and vascular invasion (P = .001). MUC4 expression inversely correlated with TTF-1 expression (P = .020) and EGFR mutations (P = .004). Interestingly, MUC4 expression correlated with HER2 protein expression (P = .042), although MUC4 expression did not correlate with HER2 DNA amplification or HER2 gene mutations. Patients with MUC4-positive tumors had significantly worse prognoses compared to patients with MUC4-negative tumors (P = .025). CONCLUSION: The present study showed that MUC4-positive lung ADCs correlated with male smokers, solid ADCs, negative TTF-1 expression, the EGFR wild-type gene, HER2 protein expression, and poorer prognoses. These results suggest that MUC4-positive lung ADC may be a distinct subtype found in patients with smoking-related poor outcomes, mediated by HER2 signaling pathway.