The clinical utilization of atrial programmed pacemakers is limited by the lead systems available for sensing of atrial activity. The endocardial method of lead placement is burdened by a dislodgement rate of up to 30 per cent. Alternatively, the patient must submit to the risks of a thoracotomy. Thirty-one patients have been treated with a transmediastinally, retrocardially positioned atrial detector electrode. In 20 patients (65 per cent) the detector performed as desired with no postimplant revision. In 11 patients (35 per cent) corrective measures were required primarily to correct lead placement; seven of these were corrected under local anesthesia merely by pulling the catheter. Spontaneous lead dislocation occurred in four patients. Four patients (13 per cent) underwent remediastinoscopy due to cranial displacement of the detector electrode with a resulting decrease in atrial potential to less than 0.5 mV. For technical reasons, the lead placements were performed without the benefit of x-ray illumination, with only an ECG check of the posterior atrial wall, and this may account for the relatively high incidence of revision. Transmediastinal placement of sensing electrodes presents a practicable alternative to methods presently used.