This study aimed to determine if the lens protein aquaporin 0 (AQP0) is present in the vitreous of pseudophakic eyes of patients presenting with chronic cystoid macular edema (CME). A case-control study was conducted. Ten patients undergoing therapeutic vitrectomy for chronic CME after uncomplicated cataract surgery were enrolled in this study. Fourteen patients with pseudophakia undergoing vitrectomy surgery for indications other than CME acted as the comparison group.The vitreous fluid from the 2 groups was analyzed for the presence of the lens protein AQP0 and type II collagen (used as a positive control). Type II collagen was detected in all the vitreous samples, whereas AQP0 was documented in 50% of eyes with chronic CME but was not found in the vitreous of any eyes without a documented history of CME. Aquaporin 0 is found in some eyes with chronic CME after uncomplicated cataract surgery, suggesting contamination of the vitreous by lens protein may have a role in the pathogenesis of this disorder.