Deprivation of growth factors (GF) induces apoptosis in vascular endothelial cells (EC). However, its precise mechanism is not clear. We studied effects of the cell density on GF deprivation -induced apoptosis. Removal of bFGF from media of a dense culture caused EC apoptosis in a time dependent fashion with the maximum value of 20% of total cells after 48 hrs, whereas removal of bFGF from a sparse culture did not. A neutralizing antibody against bFGF significantly increased the apoptosis in the sparse culture. Western blot analysis and metabolic labeling revealed that sparse EC produce larger amount of bFGF than dense EC. This difference was not due to the difference in amount of mRNA for bFGF. The study using inhibitors of proteasome, calpain and other proteases demonstrated that the difference in bFGF protein is partly attributed to greater degradation of bFGF in a dense culture by a calpain family protease(s). Our finding demonstrated that EC apoptosis is dependent on the cell-density, which affects autocrine production of bFGF by a calpain-related mechanisms. These mechanisms may have implication to maintenance of EC monolayer.