OBJECTIVE--Since 1905 there have been many reports of cases in which the right ventricle was deficient in myocardium. Several terms have been used to describe this condition. Of these, "Uhl's anomaly" and "arrhythmogenic right ventricular dysplasia" are most often used. Our study investigates the relation between these entities. METHOD--Five cases with a primary deficiency of the right ventricular musculature were examined. The findings were compared with those published reports to evaluate the similarities and differences between Uhl's anomaly and arrhythmogenic dysplasia. RESULTS--The five cases showed two patterns of myocardial deficiency in the right ventricle. On the one hand, the parietal wall was paper thin with complete absence of musculature and apposition of the endocardial and epicardial layers. On the other hand, patchy, localised fibrofatty tissue replacement was found within the parietal musculature. Evidence from our cases, combined with analysis of other publications, showed different modes and timing of clinical presentation of the patients with these two anatomical conditions, congestive heart failure or arrhythmia. CONCLUSIONS--The conditions variously described as Uhl's anomaly and arrhythmogenic dysplasia are separate and distinct morphological entities.