The aim of this study is to investigate the histopathological features of Tenon's capsule in eyes with diabetic macular oedema and to compare them between diabetic eyes and healthy subjects. The study included 26 eyes with diabetic oedema and 17 healthy eyes as healthy controls. Tenon's capsule biopsy specimens were processed with the routine electron microscopic analysis technique. Type I and III collagen fibres were labelled immunohistochemically to determine the amounts of predominating collagen fibres. Leica Q-Win program was used to calculate the amounts of collagen fibres type I and type III and independent-t test was utilized to compare the obtained results between the groups. Statistical significance was set at p < 0.05. Demographic characteristics of both groups were similar (p > 0.05). Collagen type I and type III immunoreactivity was observed both in the control and the diabetic groups. The Amounts of collagen fibres type I and type III were significantly higher in the diabetic group than in the control group (mean collagen type I area: 13.410 +/- 0.99 and mean collagen type III area: 23.692 +/- 0.17 in the control group; mean collagen type I area: 25.270 +/- 6.48 and mean collagen type III area: 28.192 +/- 0.82 in the diabetic group. p = 0.0037 for type I and p = 0.0000 for type III). In light of the findings of this study, it can be assumed that diabetes mellitus may engender increased amounts of collagen in Tenon's capsule. This alteration affecting the success of filtration surgery should be kept in mind especially in diabetic eyes with glaucoma.