An unusual mid-diastolic musical murmur developed soon after cardiac catheterization of a 55-year-old man with mitral stenosis. His physical findings consisted of an accentuated first heart sound, an opening snap and a grade 3/6 mid-diastolic rumbling murmur. No early diastolic murmur was audible. However, soon after cardiac catheterization, a mid-diastolic "cooing" murmur at a frequency of 200 cycles/sec developed. This murmur resolved with the patient in the sitting position, or by leg raising. With Valsalva or Müller maneuvers this murmur was abolished transiently, and it disappeared on administering either amyl nitrite or methoxamine. Echocardiography revealed early diastolic vibrations in the aortic valve. Pulsed Doppler echocardiograms revealed harmonic signals of the aortic cusp at a fundamental frequency of 200 cycles/sec. These harmonic signals could be recorded only in mid-diastole. The frequency patterns of the murmur and the Doppler signals were identical; therefore, the murmur was judged to be produced by aortic valve vibrations. Furthermore, resonance of cardiac structures which accentuate the murmur might be related to the occurrence of this murmur. Pulsed Doppler echocardiography is helpful in identifying the site of origin of this musical murmur.