Noma (cancrum oris, stomatitis gangrenosa) is a quickly spreading orofacial gangrene in children, caused by a combination of malnutrition, debilitation because of concomitant diseases (measles) and intraoral infections. The global incidence of noma in the world is uncertain. By comparing large numbers of noma patients and cleft lip patients in a large referral hospital for these disorders in Sokoto, Nigeria, we calculated the incidence of noma in north-west Nigeria as 6.4 per 1000 children. Extrapolation of this incidence to the developing countries bordering the Sahara Desert (the noma belt of the world) gives an incidence of 25,600 for that region and a global incidence of 30,000-40,000. Noma is a good biological parameter of extreme poverty, and hence a global monitoring system for noma can be justified. Though economic progress is the most effective preventive measure against noma, medical prevention by vaccination programmes against measles should be enhanced as well.