The vascular endothelium has a central role in the control of microvascular tone, and it has been proposed that vascular endothelial damage occurs in septic shock, producing multiorgan failure. We have developed a method of detecting circulating endothelial cells (EC) that provides direct evidence of EC shedding in human sepsis. Human umbilical vein endothelial cells (HUVEC) were seeded in whole blood and recovered by isopycnic centrifugation to validate the technique. Blood samples were subsequently taken from 11 healthy volunteers, nine ventilated intensive care unit (ICU) control patients without sepsis, eight patients with sepsis but without shock, and 15 patients with septic shock. EC were identified by indirect immunofluorescence, using antibodies to von Willebrand factor (vWf) and the vascular endothelial growth factor receptor KDR. Mean HUVEC recovery was 86% for 20 to 100 seeded cells/ml of blood. vWf-positive EC counts per milliliter were significantly higher (analysis of variance [ANOVA], p < 0.0001) in patients with sepsis (16.1 ± 2.7 [mean ± SEM]) and septic shock (30.1 ± 3.3) than in healthy (1.9 ± 0.5) or ICU controls (2.6 ± 0.6). KDR-positive EC counts per milliliter were also significantly higher (ANOVA, p < 0.0001) in patients with sepsis (4.2 ± 1.1/ml) and septic shock (10.4 ± 1.2/ml) than in healthy (0.7 ± 0.3/ml) or ICU controls (0.5 ± 0.2/ml). Cell counts made with anti-vWf antibody were consistently higher than those made with anti KDR antibody, but correlation between the two counts was high (r² = 0.93). The number of circulating KDR-positive EC was significantly higher in patients who died of septic shock than in survivors (12.0 ± 1.6/ml versus 7.1 ± 1.2/ml, p = 0.026). An increase in circulating EC can be identified during sepsis and septic shock. This supports the hypothesis that endothelial damage occurs in human sepsis.