Of 122 adult patients suspected of having rheumatic mitral stenosis, 112 fulfilled the hemodynamic and angiographic criteria for pure, isolated mitral stenosis. There were 88 females and 24 males with an age range of 16 to 60 years. The left ventriculograms (30 degrees right anterior oblique) were subjectively assessed for gross bulging of the mitral valve leaflets beyond the mitral fulcrum into the left atrium during a beat with maximal opacification. Seventeen percent of cases had typical evidence of mitral valve prolapse, which is much higher than the 3% to 5% rate reported for the general population. This phenomenon was independent of the patients' age, sex, hemodynamic findings, and/or their underlying cardiac rhythm, thus implying the direct role of rheumatic mitral stenosis in the genesis of secondary mitral valve prolapse.