Recently, complexes of matrix metalloproteinase matrix metalloproteinase-9 (MMP-9) with lipocalin-2 (neutrophil gelatinase-associated lipocalin) were found in the urine obtained from breast cancer patients, while these were completely absent in that obtained from healthy controls. In vitro data suggested a possible role for lipocalin-2 in the protection of MMP-9 against autolysis. To establish this effect in vivo, we determined the presence of MMP-9, lipocalin-2 and their complex in tumour tissue from 81 gastric cancer patients. The effect of the presence of the individual parameters, the complexes, and the inhibitors TIMP-1 and TIMP-2 on MMP-9 activity was evaluated with a bioactivity assay. Immuno-histochemical (double) staining identified epithelial cells as the most likely cellular source. Finally, evaluation of all these parameters with clinico-pathological scores revealed that tumour MMP-9/lipocalin-2 complexes were significantly related with the classifications of Laurén and WHO, and highly associated with worse survival in Cox's univariate (HR 2.087, P=0.006) and multivariate analyses (HR 2.095, P=0.025).