In a retrospective analysis we assessed occurrence, contributing factors and outcome of patients experiencing granulocytic sarcoma as a localized extramedullary relapse after allogeneic BMT. EBMT members were asked to report the number of patients transplanted for leukemia between January 1981 and December 1992 and the number of patients with granulocytic sarcoma. Of a total of 5824 patients transplanted for AML, CML or MDS by 86 teams, granulocytic sarcoma was observed in 26 patients (0.45% occurring 4-56 months after BMT. Granulocytic sarcoma occurred after allogeneic BMT in 20 out of 3071 patients grafted for AML (0.65%), and in the CML/MDS subgroup in six out of 2753 grafted patients (0.22%). Granulocytic sarcoma can involve any site of the body, presenting as a soft tissue mass; it occurred in body cavities (eg pleural cavity, abdominal cavity, spinal canal, stomach and bladder), the head and neck region (orbit, ear, skull base, peripheral nerves), the trunk and limbs, and mammary and sex glands. Granulocytic sarcoma predicts an additional hazard to outcome after BMT. Nine of 26 patients (33%) were alive 15-151 months after the onset of granulocytic sarcoma. Advanced disease stage at grafting adversely affected survival and all patients died. The best treatment option still needs to be defined.