Cystoid macular edema (CME) is a major cause of visual impairment and is thought to be due to abnormal perifoveal capillary permeability. Posterior sub-Tenon's corticosteroid injections are used to improve the visual acuity in CME, although their mechanism of action is uncertain. In this study, visual acuity, blood retinal barrier (BRB) permeability, and fluorescein angiograms were recorded immediately before and one and four weeks after the administration of steroid injections. Ten patients (12 treated eyes) with CME secondary to uveitis were studied. Visual improvement, defined as an increase in at least two lines of Snellen visual acuity, was seen in half of the treated eyes. In some patients, these improvements were not directly related to changes in the BRB permeability or the amount of macular fluid. Posterior sub-Tenon's corticosteroid injections do not consistently affect blood retinal barrier permeability.