This study is based on the Reykjavik Study, a long-term prospective cardiovascular survey, which included a representative population of 9135 men and 9627 women, 33-79 years old, who were invited to participate during the years 1967-91. Right bundle branch block (RBBB) was found in 126 men and 67 women. The prevalence increased with age, from 0% among men and women 30-39 years of age to 4.1% and 1.6% in men and women, respectively, who where 75-79 years old. The incidence increased with age. In men younger than 60 years RBBB had a significant relationship with hypertension (P < 0.05), elevated fasting blood glucose (P = 0.05), and increased heart size (P < 0.05). In men with RBBB regardless of age, an association was found with cardiomegaly (P < 0.05), ischaemic heart disease (P < 0.05), arrhythmias (P < 0.001) and bradycardia (P < 0.01). A higher mortality from heart disease (P < 0.01) was found in men with RBBB compared to the control population. This was not significant when risk factors of heart disease were taken into account by multivariate Cox analysis. There was a relationship (P < 0.05) between hypertension and RBBB in women younger than 60 years. RBBB in women younger than 60 years is often associated with hypertension and in men younger than 60 it is often associated with an underlying cardiovascular disease, hypertension, cardiomegaly and elevated blood glucose.