A case of incessant tachycardia diagnosed at the seventh month of pregnancy conditioning hydrops fetalis is described. Delivery was induced by caesarean section. Atrioventricular reciprocating tachycardia was due to the involvement of a left lateral accessory pathway refractory to multiple antiarrhythmic drugs and was often associated with ipsilateral left bundle branch block at a slower rate. To the best of our knowledge, this is the smallest premature infant in whom radiofrequency catheter ablation was necessary. There were no complaints of complications with recognizable clinical involvement but only a pericardial effusion diagnosed by echocardiography that reabsorbed within a few days. At 28 years old, the psycho-somatic development of this young man is optimal, with normal findings at cardiac imaging. This treatment modality has proven feasible but remains a procedure to be performed only in exceptional cases by very experienced operators.