Extramammary Paget's disease (EMPD) is a rare intraepidermal adenocarcinoma. The common sites of EMPD involvement are the vulva, perineal, perianal, scrotal and penile skin. Several studies have shown that HER-2/neu, also known as c-erbB-2, is amplified and overexpressed in many cancers. In this study, we investigated the expression and clinical significance of HER-2 in Japanese patients with EMPD. Keratinocytes in epidermis were slightly positive for HER-2. As for EMPD, 19 of 31 EMPD were positive for HER-2 (61%). There is significant correlation between the presence of invasion and strong positivity (3+) for HER-2 (p < 0.02). Furthermore, there is significant correlation between the presence of lymph node metastasis and strong positivity (3+) for HER-2 (p < 0.02). These results suggest that patients with EMPD strongly positive for HER-2 may have high risk for lymph node metastasis and should be followed up carefully. The observed overexpression of HER-2 in EMPD presents a potential therapeutic target for adjuvant treatment of this disease. Treatment with trastuzumab is well established in breast cancer with HER-2 overexpression and is recommended by several consensus statements. The results of the present study indicate that targeting therapies for HER-2, such as trastuzumab, may be used for EMPD particularly in patients with invasive and/or metastatic EMPD.