The aim of this study was to investigate the expression of Zonula Occludens (ZO) proteins ZO-1, ZO-2 and ZO-3, and MUPP-1 (multi-PDZ domain protein 1), peripheral/plaque proteins that function in maintaining tight junction integrity and in transducing regulatory signalling events in patients with primary breast cancer. Breast cancer primary tumours (n=114) and background tissues (n=30) were processed for quantitative-polymerase chain reaction (PCR) analysis, Western blotting and immunostaining. Standardised transcript levels of ZO-1 and MUPP-1 were significantly lower in patients with metastatic disease compared with those remaining disease-free (DF) (median follow-up 72.2 months). Immunohistochemistry confirmed these results, with decreased levels in ZO-1 staining. For both ZO-1 and ZO-3, staining was confined to the intercellular regions in normal tissue, whereas in tumour tissues staining was diffuse and cytosolic. Q-PCR revealed a reduction in the levels of ZO-1 and MUPP-1 in patients with disease recurrence. Prognostic indicators of breast cancer were also inversely correlated with ZO-1 expression. We conclude that low levels of tight junction plaque molecules, such as ZO-1 and MUPP-1, in breast cancer are associated with poor patients prognosis.