Quality of life measures are widely used in dermatology as well as in rheumatology, but there are no large studies taking arthritis into consideration when studying quality of life in psoriasis. The aim of this study was to investigate psoriasis-related quality of life in a large sample of members of the psoriasis associations from the Nordic countries including an arthritis-related evaluation. The prevalence of reported arthritis within the groups was also estimated. An Arthritis Disability Index suitable for parallel use together with Finlay's Psoriasis Disability Index was constructed. A total of 5,795 members and 702 patients seen by Nordic dermatologists rated the severity of their disease and completed the Psoriasis Disability Index formula and a Psoriasis Life Stress Inventory, and if arthritis had been diagnosed, the Arthritis Disability Index formula. Approximately 30% of all psoriatic patients, irrespective of group, received a diagnosis of arthritis either by their dermatologist or a rheumatologist. Members previously hospitalized for their disease had a higher frequency of arthritis (41%) than those without a history of hospitalization (23%). The highest prevalence of arthritis was found in Norway (33.8%). Members with arthritis exhibited greater impairment of psoriasis-related quality of life, longer disease duration, and greater self-reported disease severity for psoriasis. Important predictors for impairment of arthritis-related quality of life were pain, number of affected joints, and restriction of joint mobility. These data show, that the prevalence of arthritis in psoriasis may be significantly higher than the previously accepted average of 7%. The results demonstrate that when studying quality of life in psoriasis, arthritis and arthralgia are important independent factors to be included in the evaluation.