The connective matrix of 17 surgically excised mitral complexes from patients with clinical diagnosis of rheumatic carditis was evaluated by semi-quantitative histopathological, immunohistochemical and ultrastructural parameters. Different and concomitant patterns of loose and dense fibrosis were observed with variable constitution and organization of collagen I, III, IV, procollagen III, laminin, fibronectin and elastin. Loose fibrosis exhibited codistribution of all matrix components, Initial phase of fibrosis was characterized by deposition of all matrix components organized in a network pattern. In dense fibrosis a parallel disposition of type I collagen bundles predominated. In the denser (hyalin) fibrosis, the collagen exhibited abnormalities in fiber diameters and in fiber conformation (hyperfibers) and there was reduction or disappearance of other matrix components. The presence of these different kinds of connective matrix and the ultrastructural alterations in collagen fibers are associated to different stages of fibrosis organization and probably reflect changes in collagen susceptibility to degradation. These morphologic patterns may be related to the evolution (stability or reversibility) of rheumatic sequelae.