David G Dodwell,1 Darrel A Krimmel,1 Christopher M de Fiebre2 1Illinois Retina Center, Springfield, IL, USA; 2CMdeF Consulting, Upper Saint Clair, PA, USA Purpose: To evaluate the rate of sterile endophthalmitis (SE) following intravitreal injection of three different formulations of triamcinolone acetonide (TA) in a single physician practice and also to assess the mean diameter and concentration of particles of the two TA formulations currently available commercially in the USA. It was hypothesized that TA formulations with smaller particles and/or greater concentrations would have a higher incidence of SE.Methods: Single-site, interventional case series in which the medical records of 392 consecutive eyes receiving intravitreal TA as Triesence®, Kenalog®-40, or preservative-free TA between September 2008 and October 2013 were retrospectively reviewed for the incidence of SE. Particle sizing of TA formulations was conducted by an independent commercial laboratory.Results: Five cases of SE were identified. The four cases of SE following Triesence® (4.6%) represented a rate significantly higher than the one case of SE following preservative-free TA (0.6%; P=0.049) and the 0% incidence rate of SE following Kenalog®-40 (P=0.0210). Triesence® had significantly smaller particles than Kenalog®-40 (P<0.0001).Conclusion: The rate of SE was the highest with the formulation of TA that had the smallest particle size and highest particle load (number of particles injected). The lowest rate of SE was seen with Kenalog®-40, the only TA formulation that contained a benzyl alcohol preservative. The data do not support a principal causative role of benzyl alcohol in the development of TA-induced SE. Instead, the data support the particle theory of TA-induced SE; however, larger-scale, multicenter studies are needed to confirm and expand on these findings. Keywords: sterile endophthalmitis, benzyl alcohol, Triesence®, Kenalog®-40, preservative-free triamcinolone acetonide, particle size, frustrated phagocytosis