Dimensional alterations of the conjunctival sac during the blink may play a role in fluid elimination from or redistribution within the sac. These changes may also be important for the efficacy of certain ocular drug-delivery methods, including the retention and comfort of inserts placed in the conjunctival space. We compared 60 closed-eye and 60 open-eye polysiloxane molds of partially expanded left inferior conjunctival sacs of 10 human subjects. Overall, closed-eye molds were significantly longer horizontally by 2 mm and thicker by 0.3 mm at the midpoint of the inferior horizontal ridge below the impression of the tarsus. Volumetrically, these effects were offset by reduced vertical height of the ridge and other shape changes. Significant variation from the overall analysis was found for individual eyes. We conclude that small fluid displacements may occur within the inferior sac by tissue forces involved in eyelid closure and that the magnitude of this effect is subject-specific. Though an overall volumetric change was not substantiated for the single open-to-closed eye transition represented here, the overall difference could have become apparent upon multiple transitions as in repeated blinking. Cul-de-sac shape alterations in response to eyelid closure and other factors should be taken into account when conjunctival inserts for drug or lubricant delivery to the eye are physically designed for residence in the cul-de-sac. This project was supported in part by the Bausch & Lomb InVision Institute.