Analysis of the ecg. tracings and case records of 119 young adult males afflicted with acute rheumatic fever revealed that sinus bradycardia is present at some time during the first 5 weeks of the illness in approx. half of the cases. The appearance of the slow heart rate is correlated in time with the subsidence of fever and arthritis. The incidence of this bradycardia is not related to a past history of rheumatism, to the severity of the attack or to the use of aspirin in treatment. Those patients who manifest this period of bradycardia have a somewhat more favorable prognosis in terms of the duration of the immediate illness, and they may possibly have a more favorable ultimate prognosis in terms of residual cardiac damage.