The effectiveness and safety of balloon-tipped, flow guided, electrodes for ventricular pacing as opposed to the fluoroscopy-guided semi-rigid bipolar electrodes have never been compared in a controlled study. A prospective study was therefore undertaken to compare both techniques in semi-elective and emergency procedures. Flow guided electrodes were inserted in 67 patients (group A) and semi-rigid electrodes in 44 patients (group B). The results of group A were judged to be superior to those of group B in four aspects: a) shorter time (6'45" vs. 13'30", p less than 0.0005); b) lower incidence of catheter displacement (13.4 vs. 32.0 percent, p less than 0.05); c) longer interval of time between implantation and catheter displacement (4.4 vs. 1.9 days, p less than 0.0005); d) lower incidence of serious ventricular arrhythmias during insertion (1.5 vs. 20.4 percent, p less than 0.005). Threshold at insertion was not significantly different (0.6 +/- 0.3 vs 0.7 +/- 0.2 milliampere). The superiority of flow-guided electrodes over fluoroscopy-guided electrodes persisted in the comparison of semielective insertions in groups A and B. We conclude that the flow-guided insertion technique is safer, more expeditious and more stable than the fluoroscopy-guided technique in semi-elective as well as in emergency insertions.