Abstract We present a theory for pore dynamics of osmotically stressed vesicles. When a liposome with an internal concentration of solute is placed inside a solute-depleted medium, an osmotic flow of solvent through the lipid bilayer leads to swelling of vesicle and to increase in membrane surface tension. This can result in membrane rupture and opening of thermal pores. Depending on the internal concentration of solute and the size of the vesicle, pores can close rapidly or be long lived. We find that the life span of the long-lived pores scales non-trivially with the size of the liposome. Closure of the long-lived pore is followed by a rapid flicker-like opening and closing of short-lived pores. Our model is consistent with the observation of long-lived pores in red blood cell ghosts.