This report describes the sequence of events in the development and subsequent spontaneous resolution of functional tricuspid valve atresia in the donor fetus in a case of twin to twin transfusion syndrome. Fetoscopic laser coagulation of the placental anastomoses was performed at 20 weeks' gestation. Subsequently, there was evidence of increased placental vascular resistance in the donor twin and major impairment of right ventricular function with no forward flow through the tricuspid valve. During the next four weeks, however, there was spontaneous and complete recovery of ventricular function and resolution of the functional tricuspid valve atresia. These findings suggest that alterations in fetal haemodynamics may result in structural cardiac abnormality and may be the precursors of some forms of congenital heart disease.