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No detailed information about the ultrastructure of Tenon's capsule has been published. The purpose of the present study was to compare the ultrastructural features of collagen fibrils from Tenon's capsule in a nonstrabismic control group (seven children) to those in an infantile esotropic group (10 children). Small biopsy specimens from Tenon's capsule were taken during various operations to be examined by electron microscopy. On electron microscopy, the capsule was found to be composed of groups of collagen fibrils arranged irregularly in different orientations, forming a three-dimensional network that provides tissue resistance to stress. The cross-sectioned collagen fibrils were studied by an image analyzer. In both study groups, all fibrils had a round, regular contour. In the esotropic group, the Tenon's collagen fibrils were thicker, as reflected by their significantly greater mean diameter: 101 +/- 5 nm (mean +/- standard deviation) compared to 86 +/- 5 nm in the control group. Also, significantly greater heterogeneity was found in the collagen fibril thickness of each individual in the esotropic group compared to the control group. Moreover, the mean number of collagen fibrils per unit area was significantly higher in the esotropic group: 98 +/- 13 fibrils per 10(6) nm2 compared to 73 +/- 5 fibrils per 10(6) nm2 in the control group. These ultrastructural changes may be stress-induced secondary alterations of the Tenon's collagen fibrils resulting from prolonged deviation of the eye in infantile esotropia. The significantly denser collagen fibrils may cause a decrease in the elasticity of Tenon's capsule in infantile esotropia.