We investigated retinal vascular changes in nine patients undergoing optic nerve sheath decompression for vision-threatening papilledema associated with pseudotumor cerebri. Two major (one superior and one inferior) retinal venous and arterial calibers were measured before and 1.1 +/- 0.1 months, 3.2 +/- 0.2 months, 5.0 +/- 0.2 months, 7.6 +/- 0.2 months, and 13.0 +/- 1.9 months after surgery. The retinal venous caliber decreased significantly, and the decrease progressed until 3.2 months after surgery. The venous caliber in the contralateral unoperated eyes also decreased significantly. The decrease in the venous caliber in the contralateral unoperated eye as well as in the operated eye following the surgery supports the hypothesis that slow filtration is the mechanism of successful optic nerve sheath decompression. Retinal venous caliber measurement, in either the operated eye or the unoperated fellow eye, can be used to quantitatively monitor the status of papilledema associated with pseudotumor cerebri.