Optical and electron microscopical evidences of focal matrix degradation were frequently seen in liver sections taken from patients with periportal ("pipe-stem") fibrosis caused by schistosomiasis mansoni. Besides the presence of focal areas of rarefaction, fragmentation and dispersion of collagen fibers, the enlarged portal spaces also showed hyperplasia of elastic tissue and disarray of smooth muscle fibers following the destruction of portal vein branches. Ultrastructural changes represented by focal lytic and/or electron dense alterations of collagen fibrils were similar to those first seen in experimental material and designated as "chronic collagen degradation". Elastin and related microfibrils were also affected by focal condensation, fragmentation, distortion and dissolution. Schistosome eggs were scanty in the tissue sections examined. Matrix degradation represented involuting changes related to the progressive diminution of parasite aggression, which occurs spontaneously with age or after cure by chemotherapy. Changes of focal matrix degradation now being described represent the basic morphological counterpart of periportal fibrosis involution documented clinically, especially by ultrasonography, in patients with hepatosplenic schistosomiasis submitted to curative chemotherapy.