The incidence and the nature of medium-term complications of automatic implantable cardiac defibrillators (AICD) were studied. Seventy-nine AICD were implanted in 50 consecutive patients (42 men, aged 54.5 +/- 13.7 years). Forty-six patients had spontaneous ventricular arrhythmia. These arrhythmias were resistant to treatment (N = 9), reproducible with treatment (N = 28). In 4 patients, the indication was prophylactic, in 2 a Brugada syndrome, in 2 syncope with reinducible ventricular tachycardia and in 1 patient, torsades with a short coupling interval. Forty-six patients had underlying cardiac disease (ischaemic, N = 28, primary dilated cardiomyopathy, N = 10, others, N = 8). The ejection fraction was > 40% in 32 patients. The average follow-up was 41.3 +/- 34.9 months. Eight patients died, 2 from cardiac failure. Twenty-one patients (42%) had 1 or more complications related to their AICD. These occurred: in the operative period (N = 3): 1 post-shock atrioventricular block, 1 ruptured electrode and 1 increased threshold with amiodarone; in the postoperative period (N = 6): infection in 3 cases, cerebrovascular accident in 1 case, deep venous thrombosis of the left arm in 1 case, pneumothorax in 1 case. In the medium-term, the complications were mainly inappropriate electrical shocks observed in 14 patients related to atrial arrhythmias in 7 cases, sinus tachycardia in 1 case, over-detection of myopotentials in 2 cases and electrode dysfunction in 4 cases. In addition, the authors observed complications related to the material: AICD failure in 1 case, electrode displacement in 1 case, and electrode rupture in 3 cases. The authors conclude that AICD are effective for the treatment of malignant ventricular arrhythmias which justify strict specialist follow-up given the incidence and diversity of their complications.