OBJECTIVE Vitreous fluid that remains trapped in the peripheral vitreous cavity after vitrectomy with fluid-air exchange can result in a smaller than desired intravitreal gas bubble size. To evaluate the effectiveness of a single fluid-air exchange in dehydrating the vitreous cavity, we measured the rate and volume of posterior vitreous fluid migration after an initial fluid-air exchange. METHODS Thirty-eight eyes undergoing vitrectomy for macular hole closure had measurements of posterior vitreous fluid accumulation at either 5, 10, 15, or 20 minutes after fluid-air exchange. RESULTS An average of 0.38 ml of fluid accumulated posteriorly within 10 minutes after the initial fluid-air exchange compared to 0.22 ml that accumulated after 5 minutes (P = 0.0003). A mean of 0.04 ml accumulated during each 5-minute interval between 10 and 20 minutes after the initial fluid-air exchange. CONCLUSIONS Fluid composing 10% of the vitreous volume may migrate and accumulate posteriorly within 10 minutes of an apparently complete fluid-air exchange. Fluid aspiration after a 10-minute wait after the initial fluid-air exchange helps maximize vitreous cavity dehydration and should be employed when a large gas bubble is required after vitrectomy.