Despite psoriasis being a common skin disease, there are still a number of unanswered questions. One of these is the prevalence of the disease, as there is a lack of specific data, with the majority of studies reporting estimates only. Population based studies are rare and longitudinal observations on changing prevalence rates are lacking. This contrasts with other T-cell mediated autoimmune diseases where the number of those affected is rising. Epidemiological studies revealed that a distinct group of diseases is quite frequently associated with psoriasis, e.g. arthritis, colitis, diabetes and hypertension. In contrast, atopic dermatitis and allergies are less frequently seen compared to normal rates of occurrence. As the psoriatic immune response pattern relates to activated Th-1 cells, psoriasis and atopic dermatitis appear to be mutually exclusive due to the Th-1/Th-2 dichotomy.