Our previous study in extramammary Paget's disease showed neither p53 mutations nor allelic loss at selected loci implicated in other cancers, suggesting a pathogenesis of this skin cancer different from other common epithelial malignancies. To examine further the genetic defects in extramammary Paget's disease, we carried out molecular genetic analyses in 31 tumor samples obtained from 27 cases of extramammary Paget's disease without underlying malignancies. Immunohistochemistry using CB-11 monoclonal antibody revealed either membrane or cytoplasmic erbB-2 oncoprotein overexpression in none of the 13 primary in situ tumors, but in one recurrent in situ tumor, 10 of 13 invasive primary tumors and two of four lymph node metastases. Sensitive dual color fluorescence in situ hybridization analysis using probes for erbB-2 gene locus and chromosome 17 pericentromere, however, revealed different erbB-2 gene status in the erbB-2 overexpressing tumors. One recurrent in situ tumor and one lymph node metastasis showed definite gene amplification characterized by multiple scattered signals or a few large clustered erbB-2 signals, whereas four tumors with predominantly cytoplasmic erbB-2 overexpression were thought to have low-grade gene amplification. The remaining six tumors overexpressing erbB-2 showed no increase of erbB-2 copy numbers. No evidence of abnormal activation of the beta-catenin gene, a critical mediator of Wnt signaling pathway, in any tumor by immunohistochemical staining and by direct sequencing and reverse transcription-polymerase chain reaction analysis was found. Frequent overexpression of erbB-2 by either gene amplification or possible transcriptional activation in invasive primary tumors and metastases suggests an important part for this oncogene in the progression of extramammary Paget's disease.