Medical treatment of cystoid macular edema (CME) with carbonic anhydrase inhibitors has been known for over a decade. Initial observations were based on experimental data which suggested that acetazolamide can increase fluid absorption across the retinal pigment epithelium. Carbonic anhydrase inhibitors (CAI) have also been shown to have other direct effects both on retinal and retinal pigment epithelial cell function by inducing an acidification of the subretinal space, a decrease of the standing potential as well as an increase in retinal adhesiveness. It is thought that acidification of the subretinal space is finally responsible for the increase in fluid resorption from the retina through the RPE into the choroid. Several clinical studies have suggested that patients with cystoid macular edema due to retinitis pigmentosa and uveitis may react more favorably to CAI treatment than other etiologies such as diabetic maculopathy or macular edema after retinal vein occlusion. The present working hypothesis is that diffuse leakage from the RPE responds more readily to CAI treatment than leakage from retinal vessels. This may be due to the modulation of membrane- bound CA IV in the RPE which may have lost its polarised distribution in the presence of macular edema. A normal clinical starting dose of CAI is 500 mg/day which should be continued for at least one month to see an effect. This dose may be reduced by the patients over the course of therapy. Metaphylaxis to the drug may occur with a rebound of the edema despite continuation of treatment.