Eighteen patients with endophthalmitis involving anaerobic bacteria are presented. Endophthalmitis followed cataract surgery in seven patients, penetrating trauma in six, a corneal graft in two, and an infected filtering bleb in two; there was one case of endogenous endophthalmitis. Propionibacterium acnes was the most frequent anaerobe isolated (78% of cases). Thirty-two percent of the patients had polymicrobial infection with mixed aerobic and anaerobic species. Six cases of acute P. acnes endophthalmitis were clinically indistinguishable from other cases of mild to moderately severe endophthalmitis. Four patients presented, after cataract surgery, with chronic, low-grade endophthalmitis of 1 to 15 months' duration, emphasizing that "sterile" endophthalmitis cannot be satisfactory diagnosed clinically. The visual prognosis of treated P. acnes endophthalmitis was often good. Based on principles of anaerobic microbiology, recommendations are made for vitreous collection, transport, and culture.