In a randomly selected population screening study of 8450 men and 9039 women 33 to 71 years of age conducted in Iceland in 1967-1977, 27 men and 17 women were found to have left bundle branch bock (LBBB). The prevalence of LBBB at that time was 0.43% for men and 0.28% for women. The incidence of LBBB was 3.2 per 10,000 per year for men and 3.7 per 10,000 per year for women. All except one of 37 alive patients with LBBB were examined in 1984 including chest X-ray, echocardiography and exercise testing (Bruce protocol). Eight men had had myocardial infarction (P less than 0.05), 12 had angina pectoris, 15 had hypertension, 7 had cardiomyopathy, 13 had primary conduction disease, and 3 had pacemakers. Five men and two women had died in comparison with 18 men and 1 woman in an age-matched control group of 176 people (P : ns). Three of 5 decreased LBBB men had cardiomyopathy at autopsy. Three men died suddenly. The two women died of noncardiac causes. Only one patient in the control group had cardiomyopathy (P less than 0.01). There was no significant difference in other cardiac diagnoses between the groups. Eleven LBBB women out of thirteen had a normal exercise duration (greater than or equal to 6 min) and 11/17 men exercised normally (less than or equal to 7 min). In comparison with the control group, the LBBB patients had an increased LV diameter 2.85 +/- 0.38 vs 2.58 +/- 0.38 cm m-2 body surface area (P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)