We report a case of Epstein-Barr virus primo infection with the development of lethal haemophagocytic lymphohistiocytosis (HLH) in a 22 year-old man, who was being treated with azathioprin for colitis ulcerosa. HLH is a rare, life-threatening disease, which is caused by an inappropriate activation of the immune system with haemophagocytosis and multiorgan system dysfunction. Early initiation of treatment significantly improves survival.