Background: Behçet's disease is a multisystem disease of unknown etiology. Caspase-9 is responsible for initiating the caspase activation cascade during apoptosis. The aim of this study was to examine caspase-9 expression in both endothelial and perivascular infiltrates of patients with active Behçet's disease. Methods: Fifteen patients with active Behçet's disease, attending the First Dermatology Department, Ankara Numune Hospital, Ankara, Turkey between June 2003 and December 2005, were included in the study. Oral biopsy specimens from nine healthy volunteers were taken as the healthy control group, and skin biopsies from 18 psoriasis patients were used as the inflammatory control group. The specimens were examined with caspase-9 primary antibody. Statistical analyses were performed using SPSS 11.5. Results: The mean caspase-9-positive endothelial cell countswere 7.17 ± 2.45 in active Behçet's disease, 4.81 ± 0.76 in healthy controls, and 4.35 ± 1.34 in inflammatory controls. The difference between Behçet's disease and healthy controls was statistically significant, with increased endothelial staining in active Behçet's disease (P = 0.049). The difference between Behçet's disease and inflammatory controls was also statistically significant; the rate of staining was higher in Behçet's disease (P = 0.006). The mean caspase-9-positive dermal perivascular cell counts were 5.15 ± 2.32 in Behçet's disease, 3.32 ± 0.82 in healthy controls, and 5.54 ± 4.95 in inflammatory controls. These values did not show any statistically significant difference (P = 0.407). Conclusion: Endothelial cells are one of the key cells in Behçet's disease, and our findings support the role of endothelial cells in the etiopathogenesis of Behçet's disease. © 2007 The International Society of Dermatology.