Psoriatic arthritis (PsA) is a common form of inflammatory arthritis but is underdiagnosed. Psoriasis affects more than 1.5% of the UK population so a GP with an average list size is likely to care for around 40 patients with psoriasis. Around 15% of these patients will be diagnosed with PsA, but up to 40% of them may have evidence of arthritis if reviewed thoroughly. [With external links to the evidence base]