This study was undertaken to evaluate the effect of permanent dual-chamber cardiac pacing in hypertrophic obstructive cardiomyopathy resistant to medication, paying particular attention to atrioventricular synchrony. Sixteen patients, mean age 59 +/- 13 years (range 36 to 80 years) were divided into two groups after in initial catheter study performed under temporary VDD pacing between March 1990 and April 1993. In group I (n = 11), the gradient was decreased by more than 50% whereas in group II (n = 5), the gradient was unchanged or reduced by less than 50%. The reduction of the gradient was immediately significant in group I, the mean value falling from 104 +/- 33 mmHg (range 60 to 170 mmHg) to 25 +/- 13 mmHg (range 10 to 60 mmHg) (p < 0.0001). In group II, the gradient only decreased initially from 132 +/- 13 mmHg (range 120 to 150 mmHg) to 88 +/- 25 mmHg (range 50 to 130 mmHg) (p < 0.003) but improved atrioventricular synchrony, obtained secondarily either by pharmacological prolongation of the PR interval (association of betablocker and verapamil) or by ablation of the atrioventricular junction, improved the haemodynamic benefits. The residual gradient recorded on the 7th day was only 26 +/- 15 mmHg (range 10 to 50 mmHg) (p < 0.0001). The comparison of the two populations showed that the mean PR interval was shorter in group II (p < 0.016) and the mean value of the optimal AV Delay (the longest AV Delay with complete ventricular capture) was also lower (p = 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)