Abstract Of 184 patients with acute rheumatic fever and associated mitral insufficiency encountered during a 15 year period, 34 manifested a mid-late systolic murmur or a nonejection click, or both, during the course of follow-up. The mid-late systolic murmur later disappeared in four patients whose condition is now considered normal. In one of the four, systolic prolapse of the mitral valve was demonstrated on an angiocardiogram obtained when the systolic murmur was present. Since disappearance of the murmur there has been no evidence of systolic prolapse on meticulous echocardiographic study of the mitral valve. In another child with angiographically demonstrated systolic prolapse of the mitral valve the systolic murmur has also disappeared, but systolic prolapse is still evident on echocardiographic study. None of the 34 patients with a mid-late systolic murmur manifested the T wave abnormalities commonly associated with the familial variety of mitral valve prolapse.