Gemcitabine is a widely-used anti-cancer drug with a well-defined mechanism of action in normal and transformed epithelial cells. However, its effect on endothelial cells is largely unknown. Acid sphingomyelinase (ASMase) is highly expressed in endothelial cells, converting plasma membrane sphingomyelin to pro-apoptotic ceramide upon activation by diverse stresses. In the current study, we investigated gemcitabine impact in primary cultures of endothelial cells. We find baseline ASMase increases markedly in bovine aortic endothelial cells (BAEC) as they transit from a proliferative to a confluent growth-arrested state. Further, gemcitabine activates ASMase and induces release of a secretory ASMase form into the media only in proliferating endothelial cells. Additionally, proliferative, but not growth-arrested BAEC, are sensitive to gemcitabine-induced apoptotic death, an effect blocked by inhibiting ASMase with imipramine or by binding ceramide on the cell surface with an anti-ceramide Ab. Confluent growth-arrested BAEC can be re-sensitized to gemcitabine-induced apoptosis by provision of exogenous sphingomyelinase. A highly similar phenotype was observed in primary cultures of human coronary artery endothelial cells. These findings reveal a previously-unrecognized mechanism of gemcitabine cytotoxicity in endothelium that may well contribute to its clinical benefit, and suggest the potential for further improvement of its clinical efficacy via pharmacologic modulation of ASMase/ceramide signaling in proliferative tumor endothelium.