Despite great numbers of recent studies on immunological parameters in psoriasis, the question whether psoriasis is an immunological disease is still open. Also, it is not clear how the three main abnormalities of this disease, i.e. the association with the human leukocyte antigen system, excessive epidermal new cell production and a unique neutrophilic infiltrate within the diseased epidermis, are linked together. Analysis of large patient cohorts has now shown that two types of non-pustular psoriasis exist: one showing early onset and linkage disequilibrium for human leukocyte antigen Cw6, B13, Bw57, the other type showing late onset associated with Cw2 and B27. The pathological features of increased cell proliferation and neutrophilic inflammation are likely to be regulated by potent peptide mediators some of which are mitogenic and/or proinflammatory. There are two powerful regulatory peptides (C5ades arg and neutrophil activating peptide-1), large amounts of which have now been isolated from psoriatic scale material. Both are able to stimulate other cells to migrate and to produce further signals. The initiating agent still remains an enigma.