New data on genetics, including an extensive pedigree and certain aspects of natural history, have been compiled on Family S, which is characterized by a hereditary progressive atrioventricular (A-V) conduction defect. Concordance analysis of heart block in affected parents and their offspring suggests as a working hypothesis transmission of diathesis for the defect by means of a Mendelian autosomal dominant factor. Regression of the defect in several relatives, including reversion from first degree heart block to normal A-V conduction, defies explanation at this time. However, rapport established with the unusually large (1,067 members) and cooperative kindred will permit longitudinal evaluation of these findings.