Cattle affected by bovine Marfan's syndrome demonstrate most clinical features of the human disease, which is caused by mutations in the fibrillin-1 gene. Immunohistochemical and metabolic labeling studies in affected cattle have demonstrated abnormalities in fibrillin metabolism. Clinically identified ocular features of the bovine disease, which are similar to human Marfan's syndrome, are ectopia lentis, microspherophakia, and myopia. The purpose of this study was to compare the ocular pathology of the human and bovine diseases and to evaluate fibrillin-1 immunoreactivity in the extracellular matrix of explanted ciliary body cells from affected cattle. Eyes from affected cattle and unrelated normal cattle were examined grossly, and portions of the anterior uvea and ciliary zonule were examined by light and scanning electron microscopy. Portions of the ciliary zonular fibers were examined by transmission electron microscopy. The results were compared between affected animals and normal controls. Explanted ciliary body cells from two affected cattle and one unaffected cow were grown on chambered microscope slides, and expression of fibrillin-1 in the extracellular matrix was compared. Eyes of affected cattle were characterized by megaloglobus, increased circumlental distance, asymmetrical ciliary processes, intact but fragile zonular fibers, and ectopia lentis. Affected animals had moderately hypoplastic ciliary bodies, compact filtration angles, and long thin irises with decreased fibrous stroma. As shown by scanning electron microscopy, the zonular fibers of affected animals were wavy and loosely arranged, with abnormal sites of insertion on the lens capsule. The ciliary processes of affected animals had flattened or smooth surfaces. Explanted ciliary body cells from affected animals demonstrated decreased fibrillin immunoreactivity when compared with a normal control. The ocular pathology observed in bovine Marfan's syndrome is, in most respects, similar to that described for the human disease and will be a useful model for studies of in vivo evaluation of abnormal microfibrillar aggregation within ocular structures.