The authors report on the results of the Collaborative Bleb-related Incidence and Treatment Study (CBIITS), which was a five year multi-centre prospective study designed to investigate the incidence of bleb-related infection following filtering surgery with mitomycin C. In this study, bleb-related infection was defined as an infection that: (1) followed trabeculectomy or phaco-trabeculectomy, (2) occurred four weeks post-surgery, and (3) demonstrates clinical signs of infection related to a filtering bleb. Patients were followed up every six months for five years. A total of 1098 eyes (1098 patients) were suitable for inclusion in the study, with 824 eyes completing the five year follow-up or reached the end point of developing bleb-related infection. While 916 eyes underwent trabeculectomy alone, 182 eyes had phaco-trabeculectomy. Of these eyes, 21 developed bleb-related infection, occurring 27.3±15.9 months after surgery. The cumulative incidence of all bleb-related infection was 2.2±0.5% over the five year follow-up, while the cumulative incidence of bleb-related endophthalmitis was 1.1±0.3%. There was no difference between trabeculectomy alone or phaco-trabeculectomy, and no difference between fornix-based and limbal-based surgery. The main risk factor for infection was bleb leak (cumulative incidence 7.9±3.1%). This is the largest study to date evaluating the incidence of bleb-related infection following mitomycin C trabeculectomy. It confirms not only the relative safety of using mitomycin-C, but also the importance of long-term monitoring for bleb leak after filtering surgery.