Background: Plex-B1, the receptor of Sema4D, has been implicated in tumour growth, angiogenesis and metastasis. The binding of Sema4D to Plex-B1 can trigger the activation of Met tyrosine kinase, thereby promoting cell dissociation and invasive growth. We tested the hypothesis that the expression of Plex-B1, either alone or in association with Met, can be of predictive value for tumour progression.Methods: The expression and distribution of Plex-B1 and Met were investigated by immunohistochemistry and immunofluorescence in 50 human neoplasias originating in the breast and ovary, and correlated with clinical–pathological data at diagnosis.Results: Plex-B1 and Met were individually expressed in 14% and in 24% of the tumours, respectively. Plex-B1 and Met were co-expressed in 24/50 cases (48%), and in the majority of these (83%) Met was tyrosine phosphorylated. The expression of Plex-B1 or Met alone showed no significant correlation with tumour aggressiveness, whereas advanced stage tumours (III–IV) frequently showed Plex-B1–Met double-positive (9/13). Tumours co-expressing Plex-B1 and Met were characterised by worse grading and higher incidence of lymph node metastases. Out of 22 tumours with lymph node metastases, as many as 19 were Plex-B1 and Met double-positive (p=0.0008), and 17 expressed phosphorylated Met (p=0.002).Conclusions: Plex-B1 assumes a predictive value for unfavourable outcome when co-expressed with Met.