The aim of this study was to assess the long term prognosis of congenital atrioventricular block (AVB). From 1965 to 1990, 42 cases of congenital AVB (22 antenatal or natal diagnoses and 20 presumed congenital AVB according to Yater's criteria). The AVB was isolated in 28 cases and associated with cardiac disease in 14 cases (8 of which were corrected transposition of the great arteries). The average age of the patients was 14 years (range 32 years to 18 months) at the time of the study. There was a clear female predominance (64%). Maternal connective tissue disease was present in 18% of cases (in the group of children born after 1977 when maternal connective tissue diseases was systematically looked for). Cardiac failure was present in 10 cases (8 with associated AVB); syncope and sudden death were observed in 11 cases. The indication for pacemaker therapy was the presence of poor prognostic factors: syncope, poorly controlled cardiac failure, low heart rate, increased QRS duration, prolonged QTc, infrahisian AVB, long pauses or arrhythmias on Holter monitoring. The only significant prognostic factors in this series were a previous history of syncope, increased QRS duration and a QTc of over 0.45 seconds. Fourteen patients were paced (endocavitary pacing only from 1981), usually in the DDD mode: 8 for syncope, 2 for cardiac failure, 4 for a poor prognostic factor.(ABSTRACT TRUNCATED AT 250 WORDS)