The need for the prophylactic insertion of a pacemaker before pulmonary artery catheterization in patients with pre-existing left bundle-branch block (LBBB) is controversial. To determine the incidence of new right bundle-branch block (RBBB) and complete heart block during bedside pulmonary artery catheterization, 293 patients undergoing 307 pulmonary artery catheterizations were prospectively studied. Nine patients had pacemaker rhythms and 19 patients had an RBBB on their precatheterization ECGs and therefore were excluded from analysis. In the remaining 279 pulmonary artery catheterizations, eight (3%) were associated with the development of a new RBBB. None of the 14 patients with a pre-existing LBBB developed complete heart block. The incidence of complete heart block during pulmonary artery catheterization of patients with previous LBBB was not higher than the incidence of RBBB in patients without underlying conduction defects. Because of the rare but grave consequences of RBBB in patients with pre-existing LBBB, we recommend the use of standby external pacemakers and equipment for transvenous pacemaker insertion in these patients during pulmonary artery catheterization. We do not recommend prophylactic pacemaker insertion.