BACKGROUND AND OBJECTIVES: The optimal AV delay should allow for the completion of the atrial contraction prior to the ventricular contraction, and provide for the longest diastolic filling and maximal stroke volume. This study evaluated the effects of the AV delay on the changes in heart functions, in patients with a DDD pacemaker and normal systolic function. SUBJECTS AND METHODS: Nineteen patients were enrolled. The pacing rate was set at 70 beats/min. The AV delay was lengthened in 20-25 msec step, from 100 to 250 msec. The stroke volume and diastolic parameters were measured by echocardiography for each AV delay. RESULTS: The changes in the stroke volume associated with the changes in the AV delay were not statistically significant. However, when viewing each individual patient, the optimal AV delay for each patient could be found. When comparing the stoke volumes obtained just before, just after, and during the maximal AV delay, a statistically significant difference was found (p< 0.05). The absolute value of the E/A ratio was less than 1. The pulmonic vein flow pattern was dominated by the systolic flow, but the changes were not statistically significant. There was a tendency for the E wave propagation slope to decrease. However, the absolute value was less than 40 cm/s, and the E/E' absolute ratio was more than 8, but neither showed any significant differences. CONCLUSION: In patients with a DDD pacemaker and normal systolic function, the diastolic parameters were abnormal, and the stroke volume affected by the change in the AV delay.