Right ventricular dysplasia or cardiomyopathy is a heart muscle disorder of unknown cause that is characterised pathologically by fibrofatty replacement of the right ventricular myocardium. 1-5 Segmental right ventricular disease is usual, but evolution to more diffuse right ventricular involvement and left ventricular abnormalities with heart failure have been described. 6l0 The incidence is unknown. Clinical manifestations of the disease include structural and functional abnormalities of the right ventricle, electrocardiographic depolari-sation/repolarisation changes, andpresenta-tion with sudden death or arrhythmias of right ventricular origin. The disease is often familial (about 30%) with an autosomal dom-inant inheritance. 11 12 It remains unclear whether this genetic background predisposes to a degenerative disease with atrophy and fibrofatty replacement of the right ventricular myocardium, or whether the inflammatory cells seen in approximately 25% of cases indi-cate an infectious or possibly genetically determined immune pathogenesis." 3 Uncertainty concerning the pathogenesis of right ventricular dysplasia leads to the as yet unresolved question of whether it is a single entity or the common end point of several disease processes. The familial nature of many cases has led to recognition that in any partic-ular family the phenotypic expression of the disease can be very variable. In turn this leads to the need for criteria to delineate the spec-trum of disease that justifiably can be called right ventricular dysplasia in clinical practice. A definitive (gold standard) diagnosis of right ventricular dysplasia is based on histological demonstration of transmural fibrofatty replacement of right ventricular myocardium at either necropsy (figs 1 and 2) or surgery.'4 15 In most patients, however, assessment of transmural myocardium is not possible. Diagnosis based on right ventricular endo-myocardial biopsy specimens is inherently difficult because the segmental nature of the disease causes false negatives and because the interventricular septum is rarely involved. Biopsy specimens cannot reflect transmural changes andnot infrequently in normal sub-jects thereare islands of adipose tissue between myocytes in the right ventricle. Nevertheless the positive finding of fibrofatty