The membranes of mammalian cells are composed of an ordered array of lipids and proteins, the latter containing carbohydrate residues directed towards the exterior and important in the interaction of cells with each other and with external proteins. This external (plasma) membrane and other more simple membranes within the cell are damaged in all diseases which compromise the integrity of the cell. However, in many cases, chemical or functional changes in these membranes are central to the pathogenesis of the disease. These processes are illustrated, and a classification of membrane-related diseases is proposed. This includes: receptor-related diseases such as type II familial hypercholesterolaemia, Grave's disease, some lysosomal storage diseases and some forms of diabetes and obesity; structural instability as manifested by red cell abnormalities and multiple sclerosis; changes in lipid state as in muscular dystrophy and multiple sclerosis; altered permeability or transport as in cystic fibrosis, diseases associated with specific transport defects, and the action of many bacterial toxins, and abnormality of the cytoskeleton-membrane interface as in Chediak-Higashi disease and some diseases associated with red cell abnormalities. Different mechanisms can contribute to the membrane disorder in a single disease state and many of these are described to illustrate this diversity.