We describe a patient who presented with chronic diarrhoea, skin lesions and jaundice. Based on histopathological examinations of the affected organs combined with the clinical features, it appeared that the patient fulfilled the criteria for graft-versus-host disease (GVHD). GVHD occurs especially after allogeneic stem cell transplantation and sometimes after organ transplantations. However, this patient had never undergone such a procedure. Further examination revealed that the patient also suffered from a thymoma, which was concluded to be the cause of GVHD. Unfortunately, the patient died after resection of the thymoma. This patient is probably the second case with thymoma and full-blown GVHD and shows that GVHD can occur in the absence of a previous transplantation.