ABSTRACT.Purpose: To study the epidemiology and risk factors of early-and late-onset postoperative endophthalmitis (PE) and severe blebitis following trabeculectomy. Methods: Retrospective, single-centre, observational study with a case-control design in part. Patients sustaining PE and severe blebitis following trabeculectomy or a combined trabeculectomy with a cataract extraction procedure performed from 1990 through 2008 and diagnosed from 1990 through 2012 were recorded at St Erik Eye Hospital. Incidence data were calculated with help from the hospital records. Notes data of cases and of six randomly selected but procedure matched control patients for each case were compared. Results: The joint rate of infection was 0.46% or 34 incidents in 7402 procedures. The frequency of early (occurring <6 weeks after surgery) onset PE was 0.19%, late PE was 0.19% and severe blebitis was 0.08%. Dominating aetiologies were staphylococci and streptococci. Overall, the infection severely impaired the visual function. Combined cataract and fistulating operations were less prone to develop late infections, p = 0.04, but no other decisive factors were identified in the case-control study. Data collection for all trabeculectomy surgeries from 1998 and onward identified an increased rate for late infection with the use of mitomycin C (MMC), 8 in 1171 surgeries or 0.7%, versus no such use, 0 case of late PE in 2136 surgeries, p < 0.001. Conclusions: Postoperative endophthalmitis is a devastating complication after trabeculectomy. The use of MMC increases the risk for delayed infection. Early PE after trabeculectomy is clearly more common than PE after cataract surgery. Developing efficacious prophylactic antibiotic regimens to reduce early PE after penetrating filtering procedures should be a major priority in ophthalmic surgery.