Stromal cell-derived factor-1 (SDF-1), via its receptor CXCR4, has been implicated in metastasis of cancer, including breast cancer. Exogenous SDF-1 is known to regulate locomotion, chemotaxis and adhesion. The knowledge regarding the effect of autocrine SDF-1 on breast cancer cells is not available. The current study evaluated the effects of SDF-1 on the biological behaviour of breast cancer cells by genetically modifying the expression of SDF-1 in breast cancer cells. Two human breast cancer cell lines (MDA-MB-231 and MDA-MB-435s) and a human fetal lung fibroblast cell line (MRC5) were used. The expression of SDF-1 and the SDF-1 receptor, CXCR4 in the cell lines were studied. Expression cassettes of human SDF-1 and hammerhead ribozyme transgenes specifically targeting human SDF-1 were constructed and used to over-express SDF-1 or to knockout the expression of SDF-1 in cancer cells, respectively. Invasiveness, migration and growth of the genetically modified cells were assessed. SDF-1 was expressed in wild-type human breast cancer cell line MDA-MB-435s and fibroblast cell line MRC5, but not in MDA-MB-231 cell line. In contrast, CXCR4 expression was observed in all three cell lines tested. The ability of invasion and migration was significantly reduced in SDF-1 knockout MDA-MB-435s cells, compared with wild-type and vector control cells (p<0.01). On the other hand, SDF-1 transfected MDA-MB-231epsilonSDF1+/+ cells that stably expressed SDF-1 showed a different behaviour from MDA-MB-231SDF1+/- (plasmid control) and wild-type MDA-MB-231 cells, both being SDF-1 negative. MDA-MB-231epsilonSDF1+/+ cells displayed a higher degree of invasiveness and migration, compared with wild-type and MDA-MB-231SDF+/- cells (p<0.01). Furthermore, SDF1-knockout MDA-MB-435s cells showed a slower growth rate over a 7-day period compared with the respective control and wild-type MDA-MB-435s cells. In contrast, the growth of the SDF-1 transfected MDA-MB-231SDF1+/+ cells was markedly enhanced when compared with wild-type and vector control cells. Breast cancer cell lines, when equipped with the autocrine SDF-1-CXCR4 signal pathway, display aggressive behaviour, including an increase in invasiveness, migration together with faster growth. SDF-1, together with its receptor CXCR4 may provide important information for predicting the aggressive nature and constitute important therapeutic targets in human breast cancer.