Study of 1 family and review of 18 revealed a spectrum of inherited bradycardia from pure familial heart block (10 families), through combined familial heart block and sinus bradycardia (6 families), to pure familial sinus bradycardia (3 families). Familial heart block occurs in 2 forms, congenital (8 families) and of adult onset (8 families). Either may be associated with sinus bradycardia. Complete heart block appeared to be congenital in all 20 affected members of families with congenital block, resulting in 10 deaths, 8 within the first 10 days of life. A narrow QRS complex did not improve the prognosis of members with complete heart block. Of 17 patients with familial complete heart block of adult onset, 8 died from heart block at a mean age of 47 years. Since partial block preceded complete block in 11 subjects who had prior electrocardiograms, it may be possible to identify family members in whom complete block is likely to develop. In families with pure sinus bradycardia, sinus bradycardia was inherited with a high degree of penetrance. It resulted in nodal rhythm in all 16 affected members and in atrial fibrillation in 9. Atrial fibrillation may be protective in these circumstances, not requiring cardioversion. Sinus bradycardia in families with heart block was usually benign. Various features of formation of the conduction system may provide an embryologie basis for the combined inheritance of sinus nodal disease and heart block. Although the lesions may reside in the sinoatrial node and atrioventricular node-His bundle junction, pathologic correlates of the familial bradycardias require further elucidation.