['Vascular endothelial growth factor-A (VEGF-A) acts on endothelial cells and monocytes, 2 cell types that participate in the angiogenic and arteriogenic process in vivo. Thus far, it has not been possible to identify differences in individual responses to VEGF-A stimulation because of the lack of an ex vivo assay.', 'We report a chemotaxis assay using isolated monocytes from individual diabetic patients and from healthy, age-matched volunteers. The chemotactic response of individual monocyte preparations to VEGF-A, as mediated via Flt-1, was quantitatively assessed using a modified Boyden chamber. Although the migration of monocytes from healthy volunteers could be stimulated with VEGF-A (1 ng/mL) to a median of 148.4% of the control value (25th and 75th percentiles, 136% and 170%), monocytes from diabetic patients could not be stimulated with VEGF-A (median, 91.1% of unstimulated controls; 25th and 75th percentiles, 83% and 98%; P<0.0001). In contrast, the response of monocytes to the chemoattractant formylMetLeuPhe remained intact in diabetic patients. The VEGF-A-inducible kinase activity of Flt-1, as assessed by in vitro kinase assays, remained intact in monocytes from diabetic patients. Moreover, the serum level of VEGF-A, as assessed by immunoradiometric assay, was significantly elevated in diabetic patients.'] Is vascular endothelial growth factor-A-induced chemotaxis of monocytes attenuated in patients with diabetes mellitus : A potential predictor for the individual capacity to develop collaterals?