Summary:Accidental exposure to acute high-dose total body neutron radiation is rare. We report a 35-year-old man exposed to a total body dose of 5.4 Gy neutron-and 8.5-13 Gy ␥-radiation in a radiation criticality accident. He received a blood stem cell transplant from his HLAidentical sister. There was bone marrow recovery with complete donor chimerism. Random chromatid breaks were observed in donor cells suggesting a bystander effect of neutron exposure. The subject died 82 days after the accident (75 days post transplant) from multiorgan failure. Bone Marrow Transplantation (2002) 29, 935-939.