Intracardiac electrograms from 50 successive patients undergoing permanent pacemaker implantation have been analyzed. There were 29 male and 21 female patients aged 14 to 93 years (mean age 68.4 years). The electrograms were obtained using methods that simulated the wave form that would be detected by unipolar cardiac pacemakers. Three types of electrographic patterns were identified: qR pattern with a q/R ratio of less than 1 (type I): QR pattern with a Q/R ratio between 1 and 4.4 (type II); and Qr pattern with a Q/r ratio between 12 and 15 (type III). A type I pattern was seen in 29 patients (58 percent), type II in 18 (36 percent) and type III in 3 patients (6 percent). The duration of the follow-up period ranged from 3 weeks to 20 months (mean 9.7 months); three patients were lost to follow-up study. There were four deaths apparently unrelated to the pacemaker. Recognizable problems (either pacing or sensing failure) occurred in one patient (6 percent) with a type II pattern, in two patients (66.7 percent) with a type III pattern and in no patient with a type I pattern. On the basis of these data it is suggested that at the time of pacemaker implantation, intracardiac electrograms with a type I pattern indicating good pacing thresholds and sensing should be sought. If type II wave forms occur with good pacing thresholds and sensing then the electrode could probably be left in position. The incidence of a type III pattern is rare; when it does occur it is greatly suggestive of myocardial perforation. When this pattern is seen, the pacemaker catheter must be repositioned.