Hereditary bundle branch defect is an autosomal dominant genetic disease that, in a large Lebanese family, was mapped to the long arm of chromosome 19. Affected individuals have various combinations of conduction defects such as right bundle branch block, left or right QRS frontal-axis deviation, or atrioventricular blocks. We now further characterize this disease with the presentation of a two-decade follow-up and analysis of electrocardiographic features and mutation-carrier status. The conduction block may be overt in the first year of life, and among affected individuals, there is a worsening of the conduction block in 5% to 15% of cases, leading to complete atrioventricular block and possibly to sudden death. A group of individuals had QRS anomalies in right precordial leads such as rsr′s′, rss′, or rSr′, which may account for partial right bundle branch blocks. In this group, which we referred to as having an “r′ pattern,” 53% were actually mutation carriers, and 19% evolved toward a complete fascicular block. By contrast, mutation carriers with a normal electrocardiogram remained normal. The QRS morphologic appearance in the right precordial leads of affected individuals and r′ pattern mutation carriers is notable for the absence or weakness of negative forces resulting in a rsR′ or rR′ morphology. In addition, an r′ pattern is highly suggestive of a mutation carrier status in the presence of a broad r wave in aVR and s in V₆ or a frontal-axis deviation. Finally, mutation carriers demonstrate a conduction block significantly more often in males than females (75% and 50%, respectively). This incomplete penetrance and slow evolution suggest that the actual prevalence of hereditary bundle branch defect is very much underestimated. (Am Heart J 1997;133:249-56.)