The expression of matrix metalloproteinases (MMP) and their inhibitor TIMP-2 in serous effusions from patients with ovarian carcinoma and its association with clinico-pathological parameters were analysed. The findings in carcinoma cells in effusions were compared with corresponding primary and metastatic lesions. Sixty-six effusions and 96 tissue sections were stained for MMP-1, MMP-2 and MMP-9 applying immunohistochemistry (IHC) and analysed for MMP-2, MMP-9 and TIMP-2 expression using mRNA in situ hybridisation (ISH). MMP-2 and MMP-9 mRNA levels in 30 effusions were subsequently analysed using reverse transcription- polymerase chain reaction (RT-PCR). MMP and TIMP expression was detected in both carcinoma and mesothelial cells in effusions. The levels were consistently higher in malignant cells, significantly so for MMP-1 (P=0.016) and MMP-2 (P=0.036) proteins, as well as for TIMP-2 mRNA (P=0.008). In tissue sections, MMP-1, MMP-2 and MMP-9 protein expression was mostly localised to tumour cells, while MMP-2, MMP-9 and TIMP-2 mRNA were predominantly detected in stromal cells. Adenocarcinoma cells in effusions showed a significant upregulation of MMP-2 expression compared with primary tumours, with a concomitant downregulation of TIMP-2. RT-PCR demonstrated the presence of MMP-2 and MMP-9 in 28/30 and 0/30 specimens, respectively. MMP and TIMP are thus mainly synthesised by cancer cells in effusions, while stromal cells have a similar role in solid tumours. MMP-1 and MMP-2 production predominates over that of MMP-9 in effusions. Increased MMP-2 and reduced TIMP-2 levels are seen in ovarian carcinoma cells in effusions, possibly marking the acquisition of a metastatic phenotype.