PURPOSE--To determine the type of cardiac disease causing left bundle branch block (LBBB) in Porto Alegre, Brazil, and to assess the role of associated left axis deviation (-30 degrees or more in the frontal plane) in order to identify a specific etiology of LBBB. METHODS--Through reports from the assistant physicians or through examination of the patients, the underlying heart disease in 264 cases of LBBB was assessed. The chi-square test was used to determine a possible association between left anterior hemiblock LBBB and one or more specific type of underlying heart disease. RESULTS--Systemic arterial hypertension (30.7), ischemic heart disease (30.3), valvar heart disease (8.7), cardiomyopathies (7.5), idiopathic degenerative disease of the conduction system (1.6) and miscellaneous heart diseases (1.2) were the underlying heart diseases. The presence of LBBB did not indicate any specific type of cardiac disease. CONCLUSION--The causes of LBBB in Porto Alegre are the same as reported in the international medical literature. Upward and leftward deviation (> or = -30 degrees) of QRS axis in the frontal plane did not show statistical significant association with any type of underlying cardiac disease.