The purpose of this study was to determine the neurodevelopmental risks in patients with twin-to-twin transfusion syndrome, a rare but serious complication of monochorionic twin gestations. From a total sample of 94 twins with twin-to-twin transfusion syndrome, admitted during 1989 and 1993, 49 patients survived and 40 patients were followed to a mean age of 24 months. Neurological status and psychomotor development (Denver and Griffiths Developmental Tests) were determined. Parameters of the neonatal period were evaluated for their potential prediction. Of the 40 tested patients 18 showed a normal psychomotor development. Thirteen patients exibited a specific delay in language development and/or showed minor neurological dysfunctions. Nine twins had severe psychomotor retardation in combination with cerebral palsy. Major neurological sequelae were found, more common in recipients than in donors (6/19 vs 3/21). Correspondingly, neonatal ultrasound showed more pathological results (especially periventricular leucomalacia) in recipients. Neither anaemia nor polycythaemia at birth can predict developmental outcome. Apart from a high prenatal mortality rate, both twins, donators as well as recipients, are highly at risk for brain damage of different aetiology, associated with abnormal neonatal cerebral ultrasound.