Summary The purpose of this study was to examine the long term prognosis of ‘so-called’ incomplete right bundle branch block (RSR1 in V1 or V2) from the perspective of the risk of occurrence of complete right bundle branch block. The Manitoba Study consists of a cohort of 3983 men who have been followed with regular examinations, including ECGs since 1948. Six hundred and twenty–six cases with R1 in the right precordial leads were observed from the time of their first 12 lead ECG to the end of the observation period in 1977. Ten cases of right bundle branch block occurred in this group. Although the number with complete right bundle branch block is small, it was significantly (p This data suggests that an R1 in the right precordial leads represents a form of impaired conduction in the right bundle branch system, especially when associated with a QRS duration between 0.10 to 0.12 seconds because of the increased likelihood of complete right bundle branch block.