The authors formed a Mouth Clinic at Sunnybrook Hospital in 1973 since when there have been 3025 patient visits. Those patients with chronic ulceration present a challenge, the diagnosis sometimes being difficult and therapy not rapidly effective. The differential diagnosis includes lichen planus, pemphigus vulgaris, benign mucous membrane pemphigoid, discoid lupus erythematosus, erythema multiforme, aphthous ulcers, Behcets disease, periadenitis mucosa necrotica recurrens, specific infections and iatrogenic causes. It is possible to reach a definite diagnosis in virtually every case by means of a good history and careful clinical examination supplemented by biopsies and in some cases direct and indirect immunofluorescent studies. Treatment emphasizes scrupulous attention to oral hygiene with baking soda mouthwashes and careful teeth cleaning to minimize the accumulation of dental plaque. Specific therapy includes topical steroids in lichen planus, intra muscular gold in benign mucous membrane pemphigoid, a previously unreported treatment which considerably improved seven out of ten patients, and tetracycline mouthwashes in aphthous ulcers.