Observational cohort studies: lessons learnt from the University of Toronto Psoriatic Arthritis Program.

Observational cohorts have emerged as important resources to study course and prognosis of chronic disease, and also facilitate phenotype definition for genetic studies. We describe the development and accomplishments of the PsA cohort at the University of Toronto as an example of an observational cohort that has allowed the description of disease manifestations, course and prognosis of PsA. We also describe the development of an international collaboration based on this cohort, which will further be used to study the incidence, prevalence and predictive factors for the development of PsA among patients with psoriasis.