Paget's disease is a skin cancer characterized by characteristic (Paget) cells scattered in the epidermis. Although its prognosis is generally favorable with surgical resection, the clinical outcome turns unfavorable in cases with recurrence and metastasis. Therefore, establishment of effective therapeutic regimens is required for advanced Paget's disease. The human epidermal growth factor receptor 2 (HER2) protein, a transmembrane growth factor receptor, is frequently overexpressed in malignancies, causing activation of the phosphatidylinositol 3 kinase (PI3K) and extracellular signal-regulated kinase (ERK) signal pathways. Recently, HER2-targeting molecular therapy using trastuzumab (Herceptin; Genentech, Inc, South San Francisco, Calif) was revealed to be effective in advanced breast cancers overexpressing HER2 protein. Here, we analyzed the correlation between activation of the HER2 signal pathways and clinicopathologic parameters of 36 extramammary Paget's disease samples from 34 Japanese patients, using immunohistochemical analyses for HER2, phosphorylated HER2, phosphorylated AKT, and phosphorylated ERK proteins. We found overexpression of the HER2 protein in 19.4% (7) of the lesions, 3 of which showed HER2 amplification by chromogenic in situ hybridization. Phosphorylated HER2 protein was detected in 12 lesions (33.3%), including 2 of the 7 HER2-overexpressing lesions. Phosphorylated AKT was detected in approximately 75.0% (27/36) and phosphorylated ERK in 38.9% (14/36). Both HER2 and AKT were simultaneously phosphorylated in 9 cases (25.0%) and HER2 and ERK in 9 cases (25.0%), but all 3 molecules were phosphorylated in only 1 sample. Phosphorylated ERK correlated with the maximum diameter of the tumors (P < .025), but other immunohistochemical parameters failed to show any correlation with clinicopathologic features. These results suggest the contribution of the HER2 signaling pathway to the pathogenesis and progression of some cases of extramammary Paget's disease, for which clinical use of molecular target therapy against the HER2 pathway is warranted.