Factors determining the success of a single, low-energy, direct current-shock to achieve chronic complete heart block were studied in a consecutive group of 14 patients. A shock of 8 J was delivered by a modified device (Sirecust BSI, Siemens). If the first shock did not produce persistent complete block, multiple shocks were given. Standard 6 French U.S.C.I. catheters were used for all procedures. Chronic complete heart block was achieved with a single shock in six cases (43%; Group I); in eight others multiple shocks were needed (Group II). Neither polarity of the bipolar and of the distal His bundle recording nor the presence of sinus rhythm before the first shock were important factors to predict the outcome. The amplitude of the first recording of the bipolar His electrogram was 225±55 μV in Group 1 and 138±105 μV in Group II (P=0·029). The duration of the HV interval before the first shock was 55±12 ms in Group I and 45±11 ms in Group II (NS). It is concluded that complete permanent heart block can be achieved with a single 8-f shock in 43% of the patients. The long-term success (follow-up at least 1 month, with a range of 1 to 8) of a single 8-f shock is predicted by a large amplitude of the bipolar and distal unipolar His bundle deflection, but not by its polarity or the HV interval.