25. Rabkin, S.W., Mathewson, F.A.L., Tate, R.B.: The natural history of right bundle branch block and frontal place QRS axis in apparently healthy men. Chest 80: 191-196, 1981. We examined the characteristics, long-term follow-up, and prognosis of right bundle branch block (RBBB) detected on a routine ECG in men with no apparent heart disease. During the 29-year period, 59 cases of RBBB were observed in men with a mean age of 44.4 +1.9 years. Because marked right or left axis deviation may identify cases with concomitant involvement of the left bundle branch system, subsets of frontal place QRS (A QRS) were examined. Comparisons were made with groups of similar ages who were free of RBBB. Cases with RBBB were observed for 936 person-years (mean 15.9 +1.6 years per case), showing no excess ischemic heart disease incidence, no cases of progression to advance AV block (second- or third-degree), or sudden death. Right bundle branch block was associated with a greater proportion of both right axis (> +90 degrees) and marked left axis (-45 degrees to -90 degrees) deviation compared with those of the same age without this conduction disturbance. In apparently healthy men, RBBB had no adverse long-term prognosis regardless of frontal plane QRS axis.