Two cases with treated transposition of the great vessels and incompetence of the left atrioventricular valve showed a type B preexcitation syndrome. In one case, this consisted of a typical W.P.W. syndrome in which the second PR interval was not shortened, but rather consisted of a delta wave and a widened QRS complex. Post mortem examination showed an abnormal connection between the bundle of His and the ventricular septum, and a low insertion of the inverted tricuspid valve. The published cases of W.P.W. syndrome in cases with treated transposition are reviewed, and the mechanism of preexciation discussed in the light of the anatomical peculiarities of the malformation and of the abnormalities which are a feature of Ebstein's syndrome.