We have immunohistochemically investigated P-cadherin (P-CD) expression in a series of 210 infiltrating ductal carcinomas (IDC) in an attempt to assess the biological and prognostic relevance of P-CD in patients harboring IDCs. Although only 74/210 (35%) of IDCs expressed P-CD in >5% of tumor cells (P-CD-positive carcinomas), categorical analyses revealed that P-CD-positive IDCs were larger (26 +/- 21 cm versus 22 +/- 11 cm, P =.0568), of higher histological grade (P =.0001), and had more lymph node metastases (P =.0327) than P-CD-negative breast carcinomas. In addition, P-CD-positive tumors were negative for estrogen (P =.0001) and progesterone receptors (P =.0001) and showed reduced E-cadherin expression (P =.0276) more frequently than P-CD-negative tumors. Univariate analysis carried out in 171 patients demonstrated that P-CD expression was also an indicator of poor prognosis (chi(2) = 8.292, P =.004), extent of lymph node metastasis (chi(2) = 20.854, P =.0000), histological grade (chi(2) = 12.908, P =.0016), and negative progesterone receptors (chi(2) = 4.116, P =.042). However, only histological grade and nodal metastases emerged as independent prognostic markers in the multivariate analysis. These results suggest that although P-CD expression may be involved in the progression of IDCs, its value as an independent prognostic factor remains to be established.