It is necessary to determine if postoperative endophthalmitis is of infectious or sterile etiology. Intraocular contents should be cultured and were positive in 50% of 58 eyes tested at our institute with suspected postoperative endophthalmitis. The vitreous aspirate is more sensitive than the anterior chamber aspirate in making a diagnosis. The use of a membrane filter to concentrate vitreous samples obtained at vitrectomy increases diagnostic yield. Fifty percent of culture-positive eyes yielded gram-positive organisms. Stained smears were consistent with the cultured organism in half of the cases.