To assess the effects of chronic dual chamber pacing (DDD) on LV diastolic function in obstructive hypertrophic cardiomyopathy (HCM), 21 patients with obstructive HCM paced for refractory symptoms were studied at baseline and at 3 and 12 months. HCM patients were matched to 21 patients with obstructive HCM on conventional treatment. Left atrial fractional shortening was calculated by M-mode echocardiography; this index reflects LV end-diastolic pressure. LV outflow tract gradient decreased 65 +/- 21% with DDD pacing and the NYHA class improved (P = 0.033). Left atrial fractional shortening worsened with DDD pacing (P < 0.001). Patients with abnormal baseline left atrial fractional shortening (< 16%) were older, had a higher NYHA class, and had more severe mitral regurgitation. In this subgroup, left atrialfractional shortening did not worsen with DDD pacing and the NYHA class improved more than in patients with normal left atrialfractional shortening (P = 0.033). In conclusion, chronic DDD pacing reduces obstruction but impairs diastolic function in HCM. In patients with normal diastolic function, the untoward effects of pacing on diastolic function are more evident than in patients with abnormal diastolic function at baseline. This suggests that DDD pacing might be beneficial in a subgroup of patients with obstructive HCM and abnormal diastolic function.