The DDI mode of pacing that permits noncompetitive atrioventricular sequential bradycardia support was chosen in 65 of 480 (14%) patients selected for dual chamber pacing between February 1985 and March 1990. All patients were implanted with Pacesetter 283 or 285 pulse generators and programmed to DDI. The indications for pacing were sick sinus syndrome (n = 52), combined sinus node dysfunction and AV block (n = 13). Forty-two of these patients had a history of paroxysmal atrial arrhythmias. All patients received passive fixation atrial and ventricular leads. Follow-up thereafter was performed predischarge, and at 6 weeks, 3 and 6 months after discharge. The duration of follow-up ranged from 1-61 months (mean 31 months). Fifty-four of 65 (83%) patients chosen for DDI remain programmed in the DDI mode. Three patients were reprogrammed to VVI and eight to DDD. During the course of follow-up, six patients presented with effective VVI pacing with consistent ventriculoatrial conduction that was appropriately sensed by the atrial circuit with atrial output inhibition. A further four patients presented with "functional undersensing" due to ventricular blanking period (VBP) characteristics in these pulse generators and in this mode. Functional undersensing was eliminated in all but one patient by reprogramming the VBP to 13 msec with no subsequent episodes of provoked crosstalk inhibition. Effective VVI pacing was observed in patients with AV block during times of sinus acceleration.(ABSTRACT TRUNCATED AT 250 WORDS)