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, alpha-catenin, and beta-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.