Vascular endothelial growth factor (VEGF-A) was first described in 1989 for its angiogenic and mitogenic properties. Early studies indicated that VEGF-A acts primarily in a paracrine pathway which is limited to vascular endothelium. Further investigation showed that VEGF-A and VEGF receptor-2 (VEGFR-2) are expressed by many solid tumors and improve cell growth and survival. Therefore, VEGF-A may act via an autocrine pathway that effects tumor cellular proliferation by binding VEGFR-2 at the cell surface. This study utilizes small interfering RNA (siRNA) technology to investigate the presence of an autocrine loop in human RKO colorectal cancer cells. RT-PCR demonstrated the expression of VEGF-A, VEGF-B, VEGF-D, placental growth factor (PlGF), VEGFR-2, neuropilin-1 (NP-1) and neuropilin-2 (NP-2) in vitro by RKO cells. Transfection with siRNA against VEGF-A resulted in a 94% knockdown of VEGF-A expression by ELISA. Northern blot, quantitative real time PCR and semiquantitative RT-PCR confirmed the knockdown data. In addition, transfected RKO cells showed a 67% decrease in cellular proliferation by WST-1 assay. This data correlated to the ELISA results. In summary, the presence of VEGF-A and VEGFR-2 argues in favor of an autocrine loop in human colorectal cancer cells. siRNA targeting of VEGF-A remains a promising anti-tumor therapeutic strategy.