To determine the hemodynamic effects of postoperative left bundle branch block (LBBB) in patients with hypertrophic obstructive cardiomyopathy (HOCM), we investigated 28 patients using Swan-Ganz pulmonary artery catheterization at rest and on exercise. Fourteen patients had postoperative LBBB (group A) and 14 had undisturbed intraventricular conduction (group B). All patients were examined by clinical investigation, electrocardiogram and bicycle ergometer exercise preoperatively and postoperatively (mean 6 months). Pulmonary artery pressure and pulmonary capillary wedge pressure were continuously measured, stroke volume index and cardiac index were obtained by the thermodilution method. All patients showed an improvement of their clinical symptoms (NYHA class: 2.8 +/- 0.45 before and 1.7 +/- 0.22 after operation) (P < 0.05). The postoperative exercise capacity (Watt) increased significantly (P < 0.05) in group A by 38% and in group B by 30%. The maximum mean pulmonary artery pressure on physical exercise decreased in group A from 40.7 +/- 9.1 to 32.5 +/- 8.7 mmHg and in group B from 42.8 +/- 12.4 to 32.4 +/- 9.2 mmHg (P < 0.05). The maximal stroke volume index and cardiac index improved significantly in 9 of 14 patients in each group. Thus, patients with HOCM and LBBB after myectomy have a marked improvement in postoperative hemodynamics, the results equaling those of patients with undisturbed intraventricular conduction.