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Micropapillary differentiation in adenocarcinomas has recently been associated with poor prognosis because these tumors are more likely to metastasize. However, no clear explanation exists as to why the presence of a micropapillary pattern is associated with metastasis. A case of primary lung adenocarcinoma with a prominent micropapillary pattern is presented here, with special reference to the immunohistochemical expression of the E-cadherin-mediated system and IQGAP1. Histologically, the tumor was diagnosed as a moderately differentiated papillary adenocarcinoma, showing an extensive micropapillary pattern, with intrapulmonary metastases, pulmonary disseminations, lymphovascular invasions, and lymph node metastases. Immunohistochemically, positive staining for the adhesion molecules E-cadherin, α-catenin, and β-catenin was detected in both the micropapillary and non-micropapillary areas, whereas IQGAP1 was detected in the micropapillary, but not in the non-micropapillary, area. The adhesive function of E-cadherin depends on the integrity of the entire cadherin–catenin–actin network, and thus the expression of IQGAP1 may lead to adherens junction disassembly, and consequently, the release of carcinoma cells organizing in a micropapillary pattern. This is the first report to suggest correlation between adenocarcinoma with a micropapillary pattern and the presence of adhesion molecules, and offers an intriguing first glimpse on the role of the micropapillary pattern in the process of metastasis.