Clinical management of patients with symptoms caused by pharmacological refractory hypertrophic obstructive cardiomyopathy must consider surgical myectomy, percutaneous transluminal septal myocardial ablation and implantation of a DDD pacemaker. Until now, no prospective, double blind, randomized studies have yet been carried out to determine the merits of each of these treatment alternatives. However, uncontrolled studies have shown that short atrioventricular delay dual-chamber pacing reduces outflow tract obstruction. Aim of the study was to investigate the results of the pacemaker therapy in a prospective, double blind randomized crossover procedure. All patients with hypertrophic obstructive cardiomyopathy included in this multicenter study were either refractory or intolerant to drugs and typically had pressure gradients higher than 30 mm Hg. In 83 patients, mean age 53 (18 to 82) years who responded favorably to a temporary pacing test, a DDD pacemaker was implanted. After echo- and echo-Doppler-based measurements of hemodynamic parameters the patients were randomized into 2 groups, those with an implanted pacemaker in the inactivated mode (AAI) and those with a pacemaker in the activated mode (DDD with optimized short AV delay). A crossover of these groups was performed after 12 and 24 weeks, respectively. Both objective parameters of echo including Doppler, and spiroergometry, and subjective parameters of angina, dyspnea, and quality of life were recorded. Additionally, subgroups based on age decades were analyzed. After 12 weeks in the DDD mode, regardless of the randomization sequence, a decrease of the pressure gradient from 59 +/- 36 mm Hg (median) to 30 +/- 25 mm Hg was proven significant (p < 0.001). The endurance of the patients who, during screening, achieved less than 10 minutes of exercise by the Bruce protocol improved by 21% under DDD mode. The main symptoms, as measured by the NYHA classification, improved statistically significant from a mean of 2.4 to 1.7 for functional class, from a mean of 2.4 to 1.4 for dyspnea and from a mean of 1.0 to 0.4 for angina. Subgroup analysis showed improvements depended significantly upon age, with a marked improvement between the ages of 60 and 70, which was statistically significant as compared to other decades. Subjective improvements in the quality-of-life of patients was measured using a specially developed questionnaire. These findings justify, by all means, the intention to implant a DDD pacemaker in older patients. In younger and/or such patients with elevated pressure gradients, the results of ongoing randomized studies comparing myectomy, PTSMA and pacing have to be considered.