Mast cell leukemia (MCL) is a very rare subtype of systemic mastocytosis, characterized by the leukemic expansion of immature mast cells. We present a case of MCL with extramedullary involvement of cervical lymph node and lung demonstrated by the initial F-FDG PET/CT scan. After a transient complete remission by induction chemotherapy and allogeneic hematopoietic stem cell transplantation, the follow-up PET/CT showed extensive extramedullary relapse involving multiple lymph nodes and small bowel. F-FDG PET/CT may be a useful tool to comprehensively stage and follow-up MCL.