The medical literature in the past two to three years has contained articles concerned with the reactivation of rheumatic fever following mitral commissurotomy. It is the purpose of this paper to present the essential findings and conclusions following the study of 100 consecutive patients who received surgery for mitral stenosis in relation to the clinical and laboratory findings that might suggest reactivation of rheumatic fever. Of the 100 patients 4 might be considered as having reactivation of their rheumatic fever. Preoperatively we attempt to have the patient free of active rheumatic fever and other nonrelated infections. We find that blood studies are important and helpful but not absolute. The sedimentation rate and antistreptolysin titer have been normal, yet the microscopic studies of the auricular appendage may reveal a marked rheumatic activity. The patient must be well compensated and the circulating blood cells within normal limits as to number and quality