Eighty cases of a well defined myocardial syndrome, an important feature of which is the interruption of T waves by premature QRS complexes, are reported. Other features of the syndrome include myocardial damage and sudden death, multiform premature ventricular complexes, aberration in the form of ventricular complexes of supraventricular origin, variation in the interval between successive ectopic complexes and the preceding sinus complex and ectopic complexes in runs of two or more. Post-extrasystolic T wave changes are sometimes observed. The R on T phenomenon is by no means uncommon when looked for, and it may be seen in a wide variety of pathologic states involving the heart, such as myocardial infarction, ischemic heart disease, hypertensive heart disease and several distinct myocardiopathies. It may also be seen during cardiac surgery and during diagnostic procedures such as cardiac catheterization, angiocardiography and ventricular puncture. The phenomenon may be suspected clinically when very premature beats are noted on auscultation, and it is frequently found in electrocardiograms recording such arrhythmias as multiform ventricular systoles, ventricular tachycardia, flutter and fibrillation, auricular paroxysmal ftachycardia and auricular fibrillation. The incomplete syndrome without T wave interruption is very common and, frequently, further observation and electrocardiography will reveal the missing features. The syndrome is frequently associated with sudden death and prophylactic administration of quinidine is recommended as a therapeutic measure.