OBJECTIVE:To evaluate the therapeutic effect of acetazolamide treatment on chronic cystoid macular edema (CME). METHODS:Thirty-seven patients (40 eyes)with documented chronic CME of various causes were prospectively treated for 4-week periods with acetazolamide or a placebo in a randomised,crossover study that compared their effects on the reduction of macular edema and improvement of visual functions. Central retinal artery(CRA) blood flow was determined using Doppler velocimetry and vessel diameter measurement using computerised digital image analysis of monochromatic fundus photographs on 10 patients (12 eyes)with CME pre-and post-administration of acetazolamide. RESULTS:More than half the patients showed a response to acetazolamide with partial or complete resolution of. edema. Thirty eyes had improved in visual acuity in the treated patients. Statistically significant improvement were seen in 10deg; thresholds of visual field and ERG b wave amplitude. No improvement was seen when the patients received placebo. There were significant increase of blood flow velocity in CRA and retinal vessels diameter after acetazolamide were administered 3 hours later in CME patients (Plt;0.05). CONCLUSION: Acetazolamide could be of value in reducing the degree of edema and improving visual function to chronic CME. The improvement on retinal circulation might be a major cause to limit the CME. （Chin J Ocul Fundus Dis,1997,13: 89-92）