Hyaluronectin (HN) is a glycoprotein with a high affinity to hyaluronic acid (HA) and known to be a component of the extracellular matrix of tumours. Clinical studies have shown that a low level of HN correlates to tumours with poor prognosis, whereas a high level of HN correlates to tumours with good prognosis. We previously demonstrated in vitro that hyaluronidase activity, which promotes tumour progression and metastatic spread by degradation of HA into angiogenic oligosaccharides, was inhibited or promoted by HN, according to the level of HN-expression. This raises the question of the role played by HN in cancer, and particularly if high and low levels of HN-expression could trigger opposite effects on tumour growth and/or metastatic spread. To address this issue, we used a model of spontaneous lung fluorescent metastases that we characterised previously. We stably transfected the human HN cDNA into fluorescent H460MGFP cells and selected two clones characterised by different levels of HN-expression: HN110 and HN704, with a high and a low level of HN-expression, respectively. In vitro, we demonstrated that HN704 cell migration was significantly increased. Inoculation of clones to nude mice had no significant effect on tumour growth, but clearly revealed opposite effects on metastatic spread: HN110 significantly decreased the number of fluorescent metastases whereas HN704 significantly increased it. We also analysed HN, HA and hyaluronidase contents in sera and tumours. These results demonstrate that HN can play a role as either a suppressor or promoter of metastatic spread.