Invasive lobular carcinoma (ILC) is more frequently lymph node positive than is invasive ductal carcinoma (IDC), and ILC cell infiltration shows distinctive histological characteristics, suggesting the action of ILC-specific invasion molecules. To identify such a molecule, we used a proteomic approach in the pseudopodia of MDA-MB-231 breast cancer cells. A pseudopodial constituent was identified using excimer laser ablation, two-dimensional difference gel electrophoresis, mass spectroscopy, and immunocytofluorescence. MDA-MB-231 cells were modified to express various levels of this constituent by transient transfection and were examined for pseudopodia formation and migratory abilities using wound healing and two-chamber assays. Immunohistochemical positivity of human breast cancer cells (56 ILCs and 21 IDCs) was compared with clinicopathological variables. An actin-binding adaptor protein, α-parvin, was found to localize to pseudopodia and to form focal adhesions in cells not induced to extend pseudopodia. Pseudopodial length and density and migratory abilities correlated with α-parvin expression. Twenty-one (37.5 %) ILCs stained positive for α-parvin, whereas the results were negative for all 21 IDCs (P < 0.001). α-Parvin positivity in ILC was significantly associated with lymphatic invasion (P = 0.038) and lymph node metastasis (P = 0.003) in univariate analyses and to lymph node metastasis (P = 0.020) in multivariate analyses. α-Parvin, a pseudopodial constituent, was found to promote migration of breast cancer cells and to be expressed exclusively by ILC, suggesting that α-parvin is an ILC-specific invasion molecule that may have clinical utility as a biomarker for aggressive subsets of ILC.