Paget's disease of the nipple (PDN) is characterized by Paget's cells infiltrating from an underlying breast carcinoma into the epidermis of the nipple. The tumor cells were reported to overexpress ERBB-2 protein. There is no evidence, whether the overexpression is caused by amplification of the ERBB2 gene (as is the case of invasive duct carcinomas) or by other causes. Because the Paget's cells proliferate vividly we were interested also in cyclin D1 expression and in its relation to the number of CCND1 gene copies. To address these issues, we performed a study using fluorescence in situ hybridization on interphasic nuclei (I-FISH) on formalin fixed / paraffin embedded tissue sections from twelve women with PDN. We compared immunohistochemical (IHC) expression of the ERBB-2 protein and cyclin D1 with the copy number of the gene ERBB2 and CCND1, respectively (I-FISH). In all cases of our series we found overexpression of the ERBB-2 protein (3+), and in all of them there was a strong amplification of the ERBB2 gene (more than 10 signals per a nucleus, usually about 20). Keratinocytes of the epidermis had two signals of the gene. IHC and I-FISH revealed comparable findings in successive biopsies in two patients. Cyclin D1 was positive in seven cases. We found a moderate amplification of the CCND1 gene (up to 10 signals per a nucleus) in three patients only. Any correlation between the cyclin D1 overexpression anda copy number of the CCND1 gene was observed. Combined numerical changes of chromosome 17 and of chromosome 11 were found in seven women. It is concluded that in PDN, the ERBB-2 protein overexpression is caused by amplification of the ERBB2 gene. A specific immuno-therapy with trastuzumab (Herceptin) used in patients with invasive duct carcinoma may be also effective in patients with PDN.