There is an increasing evidence of the crucial role of angiogenesis in cancer, and randomized studies have indicated that a "pure" anti-angiogenic drug (the anti-VEGF antibody Avastin) is very effective in colorectal cancer. In addition, this and other anti-angiogenic drugs have demonstrated activity and are currently under clinical investigation in a variety of other cancer types. At the present time, however, there is a scarcity of useful endpoints for treatment outcome beside survival. Using flow cytometry, quantitative PCR and cell culture we have found that circulating endothelial cells and progenitors are increased in cancer patients, and that measuring their viability and kinetics may offer significant clinical insight in the management of cancer patients.