Objective. The purpose of this study was to analyze the ultrastructure of gingival connective tissue from patients in one family affected by hereditary gingival fibromatosis (HGF). Study design. Electron microscopic examination was performed with gingival tissue from 10 patients from a Brazilian family with 132 members. Fifty of 96 persons at risk for this disorder were affected, which is consistent with an autosomal dominant pattern of inheritance. Results. The extracellular matrix showed flocculent material and collagen fibrils with structural abnormalities and variation in diameter. Increased numbers of oxytalan fibers were identified; however, elastic fibers were rare in the analyzed areas. Conclusions. The structural alterations found in HGF appear similar to those described in certain other heritable collagen disorders, suggesting that HGF should be included in the group of hereditary diseases in which connective tissue alterations have a distinct pattern, in contrast to reactive fibrotic gingival enlargements with no genetic component.9217882