Patients presenting with recurrent orogenital ulcers may have complex aphthosis, Behçet's disease, secondary complex aphthosis (e.g. Reiter's syndrome, Crohn's disease, cyclical neutropenia) or non-aphthous disease (including bullous disorders, erythema multiforme, erosive lichen planus). Behçet's syndrome is a multi-system vasculitis of unknown aetiology for which there is no diagnostic test. Diagnosis is based on agreed clinical criteria that require recurrent oral ulcers and two of the following: recurrent genital ulcers, ocular inflammation, defined skin lesions and pathergy. The condition can present with a variety of symptoms, hence a high index of suspicion is necessary. The most common presentation is with recurrent mouth ulcers, often with genital ulcers; however, it may take some years before diagnostic criteria are met. All patients with idiopathic orogenital ulcers should be kept under review, with periodic focused assessment to detect evolution into Behçet's disease. There is often a delay of several years between patients fulfilling diagnostic criteria and a diagnosis being made, which may contribute to the morbidity of this condition. Despite considerable research effort, the aetiology and pathogenesis of this condition remains enigmatic.