In October 1992, severe dilatation of the right atrium was detected in a 35 week foetus. The septal leaflet of the tricuspid valve seemed to be displaced distally causing massive tricuspid regurgitation. The diagnosis of Ebstein's anomaly was made and confirmed after birth. Refractory right heart failure occurred at the age of 10 months and the little girl was operated. At surgery, the right atrium was very dilated; the tricuspid valve was normally positioned and was normally constituted. The right atrial wall, partially resected, contained few muscular fibres and showed patchy fibroelastosis. Four years after surgery, the child remains asymptomatic. Dilatation of the right atrium and the tricuspid annulus caused pseudo-displacement of the septal leaflet of the tricuspid valve. This, combined with the importance of tricuspid regurgitation, led to the erroneous diagnosis of Ebstein's anomaly. It is important to differentiate idiopathic right atrial dilatation from Ebstein's anomaly because surgery is much more difficult in the latter case.