A 78-year-old man came for a scheduled check up one month after the implantation of a permanent dualchamber pacemaker for symptomatic transient atrioventricular conduction disturbances (intermittent Mobitz II atrioventricular block). The patient's ECG indicated a loss of atrial capture by the atrial electrode with 1:1 atrioventricular conduction. A subsequent X-ray examination showed that both electrodes were well placed and in their correct sites. A detailed check of the pacemaker using the programmer, together with an echocardiographic examination, revealed the true nature of the malfunction: there was a significant delay between the atrial capture and atrial depolarisation and systole, as well as a loss of ventricular capture because of an acute increase in the threshold. Normal pacemaker function and pacing ECG were restored through modification of the pacemaker's functional parameters.