Abnormalities of cardiac rhythm have been reported by others in patients with a persistent left superior vena cava. We present the histological findings from postmortem examination of the cardiac conduction system of two such patients. The first patient was a schoolboy who died suddenly and unexpectedly. His sinus node was abnormally small, his A-V node contained numerous venous lacunae and was stretched out beneath the enlarged coronary sinus, both A-V node and His bundle were dispersed in the central fibrous body in a fetal pattern, and isolated fragments of A-V nodal tissue were connected to the myocardium at the crest of the interventricular septum. The second patient complained of palpitations many months prior to surgical correction of an interventricular septal defect; his postoperative course included multiple arrhythmias and he died suddenly on the 16th postoperative day. Although his sinus node was histologically normal, the nutrient artery contained a polypoid fibromuscular mass virtually occluding its lumen; his A-V node and His bundle were also dispersed in the central fibrous body in the fetal pattern, and the A-V node contained numerous venous lacunae as well as being histologically disorganized in its cytological pattern. These anatomical findings may indicate a basis for various forms of cardiac electrical instability, and deserve consideration in the future evaluation of any patient found to have a persistent left superior vene cava, but particularly if there is clinical evidence suggesting an arrhythmia or conduction disturbance.