The term peritoneal sclerosis simply means the presence of sclerotic tissue in the peritoneum. Qualitative and quantitative morphological aspects indicate that there are two nosological entities: simple sclerosis and sclerosing peritonitis. Simple sclerosis is a thin (<40-50 microm) layer of submesothelial sclerotic tissue often limited to certain peritoneal areas, with monotonous histology. It is a component of the slight anatomical alterations constantly detectable in peritoneal dialysis patients. Sclerosing peritonitis is characterized by very thick (1,0004,000 microm) sclerotic tissue involving the whole peritoneal wall, often with inflammatory infiltrates, microabscesses, giant cells of macrophagic origin, calcifications and severe vascular alterations. Intermediate stages between simple sclerosis and sclerosing peritonitis have rarely been detected. Simple sclerosis and sclerosing peritonitis also seem to be distinct with respect to frequency, etiology, reproducibility in animal models and clinical manifestations.