Some workers state that sinus rhythm is essential for electrocardiographic placement of central venous catheters. We performed a prospective study to compare location control by ECG and by chest X-ray in 40 patients with absolute arrhythmia and atrial fibrillation. The criteria accepted as allowing the assumption of an intracardiac position of the catheter tip were: (1) Abrupt appearance of high-voltage P-waves when the right atrium (RA) was entered and their brisk disappearance when pulling the catheter back into the vena cava superior (VCS) and/or (2) a change in configuration and voltage of the QRS complex on withdrawal of the catheter from the right ventricle (RV). After establishment of an intracardiac position, the catheter was withdrawn until the ECG changed to show a trace identical to that seen before it had entered the heart. Then, in this study, the correct central venous position was confirmed by chest X-ray. The intravascular ECG revealed a correct placement of the catheter tip in the VCS in all patients but one. In this patient who had severe dysrhythmia, an intracardiac ECG could not be obtained, although the chest X-ray showed a correct position of the catheter in the VCS. While false-negative results (where an intracardiac catheter position cannot be documented although the catheter is in a central venous position) occasionally do occur, false-positive results (with ECG suggesting an intracardiac location read, though the catheter tip is actually in a peripheral vein) are virtually impossible.(ABSTRACT TRUNCATED AT 250 WORDS)