Subconjunctival gentamicin was identified as the cause of conjunctival chemosis and capillary closure in a recent study conducted in this department. The gentamicin preparation used in the study contained preservatives. The current prospective study was set up to investigate the role of preservatives in the conjunctival toxicity of subconjunctival gentamicin. Seventy five patients undergoing cataract surgery were enrolled in the study. They were split into three groups of 25 each. Group A patients were given a subconjunctival injection of a preservative-free aqueous solution of gentamicin at the end of the cataract procedure. Group B patients were given a subconjunctival injection of gentamicin containing sodium metabisulphite and disodium edetate as preservatives at the end of the cataract procedure. Group C was the control group where patients were not given any subconjunctival injection. The incidence of severity of conjunctival chemosis were observed in the three groups. The difference between groups A and B patients who received preservative-free gentamicin and gentamicin with preservatives respectively was significant (p < 0.02).