OBJECTIVE: To document co-twin death/pregnancy loss and brain injury after single intrauterine death (sIUD) in monochorionic pregnancies. DESIGN: A total of 135 pregnancies with sIUD were reviewed for co-twin IUD, miscarriage and abnormal antenatal and postnatal neuro-imaging. SETTING: A tertiary referral fetal medicine unit from 2000 to 2007. POPULATION OR SAMPLE: All cases referred with a single fetal death in monochorionic pregnancy, including those where sIUD was spontaneous or occurred after fetoscopic laser treatment, or resulted from selective termination by cord occlusion with bipolar diathermy or intrafetal vascular ablation with interstitial laser. METHODS: Clinical details and ultrasound findings of the study population were retrieved from ultrasound and institutional databases. Delivery and neonatal outcome data were obtained from discharge summaries supplemented by individual chart review. MAIN OUTCOME MEASURES: Co-twin death or pregnancy loss and neurologic injury assessed on antenatal ultrasound and MR-imaging. RESULTS: A total of 81 sIUDs resulted from vascular occlusive feticide (diathermy or interstitial laser), 22 followed placental laser and 32 were spontaneous. In 22 pregnancies (16.8%), the co-twin died in utero and eight pregnancies miscarried (6.1%). Antenatal magnetic resonance (MR) imaging in 76/91 (83.5%) continuing pregnancies detected antenatal brain injury in five (6.6%). Three infants (two not scanned antenatally) had abnormalities detected postnatally. Brain abnormality was detected less often after procedure related (2.6%, 2/77) than spontaneous sIUD (22.2%, 6/27, P = 0.003) and after early compared with late gestation sIUD (3.6%, 4/111 versus 20.0%, 4/20; P = 0.02). CONCLUSIONS: We confirm substantial co-twin loss (22.9%) after monochorionic sIUD, but a low risk of antenatally acquired MRI-identified brain injury, suggesting this risk has been overestimated. Procedures restricting inter-twin transfusion reduce, but do not negate risk of brain injury.