This study analyzes the possibility of using an oesophageal lead in the ambulatory ECG monitoring in order to improve the diagnostic effectiveness of the method by reliable identification of atrial activity. The oesophageal Holter monitoring was performed on 19 patients (pts) (8 F and 11 M, aged 34 to 79 years). These patients posed precise diagnostic problems unsolved by previous conventional Holter recordings. The oesophageal Holter recording was carried out with a Spacelabs recorder, model 90205 with two channels. One electrode was positioned in the standard lead CM5 and the second in the oesophagus using an Esodyn 2 electrode with a dynamic configuration made by C.B-Bioelettronica (Calenzano, Florence). The oesophageal lead was connected with the recorder by filter model EHF-4. The electrode was swallowed by the patient and the best position regulated on the P diphasic deflection of the highest amplitude. The monitoring allowed correct identification of the P wave over a 24-hour period. The arrhythmia which required the oesophageal Holter monitoring was recorded again in 15 of 18 patients examined. In each of these cases it was possible to solve the diagnostic question. The following arrhythmias were found: constant time correlation between the P deflection and the QRS in accordance with a supraventricular tachycardia with intraventricular conduction delay (4 pts); A-V dissociation in accordance with a ventricular tachycardia (2 pts); pause caused by atrioventricular block 2:1 (1 pt), second degree sinus-atrial block (1 pt), blocked premature atrial beats (4 pts); premature ventricular and premature atrial beats with intraventricular conduction delay (3 pts).(ABSTRACT TRUNCATED AT 250 WORDS)