Ten patients with bilateral nanophthalmos underwent sclerectomies for uveal effusion. Ultrastructural examination of the sclera revealed abnormal collagen in seven patients. Four showed dramatic fraying of the collagen fibrils into fine filaments 2 to 3 nm in diameter. In three of these cases and three other cases without fraying, there were foci of 10- to 35-nm small collagen fibrils, some appearing to arise by splitting of otherwise normal collagen fibrils. In areas of fraying, elastic fibers were absent. All patients had a wider range of collagen diameters than did control subjects. The youngest patient with fraying also had Hallermann-Streiff syndrome. In three patients, no collagen abnormality was found. The clinical feature correlating best with the presence of abnormal collagen was an extremely small eye, since the three patients without collagen abnormality had the largest eyes (range of anteroposterior diameters, 19.2 to 20.3 mm). Nanophthalmos appears to result from several distinct defects.