This study has been carried out in order to understand and increase our knowledge about the oral manifestations and the treatment required for Graft Versus Host Disease. The complication of allogeneic transplants with haematopoietic cells is revised. The clinical case report deals with a nine-year-old girl who was treated in our unit of Integrated Paediatric Dentistry after referral by her paediatrician. The symptoms of Graft Versus Host Disease are basically characterised by immunodeficiency, diarrhoea, weight loss, dermatological and hepatic alterations and oral manifestations. This group of syndromes, caused by the fact that the receptor acquires 30% to 50% of the donor's immune system, can become acute leading to a 20% to 30% mortality rate. The syndromes are chronic in 50% of cases, when sometimes the destruction of the salivary glands is the only manifestation and has a mortality rate of 5%. From the odontological point of view 80% of patients are affected, although we should make the distinction between the manifestations which are directly caused by the disease, and those caused by chemotherapy or radiotherapy. Oral manifestations (caries, mucositis, ulcers, infections, etc) should be treated as soon as possible. Positive co-operation between physicians is of vital importance.