Rapid intraoperative choroidal effusion and flattening of the anterior chamber occurred during glaucoma filtering surgery in four young patients with prominent episcleral vessels, elevated episcleral venous pressure, and advanced open-angle glaucoma. Two of these four patients had Sturge-Weber syndrome. Intraoperative release of suprachoroidal fluid (SCF) through a posterior sclerotomy facilitated reformation of the anterior chamber and repositioning of the iris and ciliary body. Posterior sclerostomy performed prior to opening the anterior chamber minimized the above-mentioned untoward series of events. Analysis of SCF and serum demonstrated considerable differences in total protein and individual immunoglobulin levels; this appears to be a manifestation of molecular sieving at the level of the choriocapillaris. Choroidal detachment and postoperative serous retinal detachment are manifestation of this phenomenon.