Left ventricular myocardial biopsies were performed during surgery in 11 patients with pure and isolated mitral stenosis. Patients had undergone a preoperative angiocardiographic study of left ventricular function. Biopsy specimens were examined with the light and electron microscope. Myocyte cell diameter was normal (20 +/- 1.6 mu). Lesions existed which were probably degenerative, including anarchy and irregularities of sarcomeres, Nemaline Myopathy-type Z line changes and alterations of intercalated discs. A moderate fibrosis was found in the interstitial spaces with very few histiocytes. The coincidence planimetry study of the interstitial spaces showed a 37 +/- 5.5% increase compared to a control group with no fibrosis (23 +/- 1.5%, p less than 0.01). The angiocardiographic indices of left ventricular function were all decreased. Only four subjects had normal left ventricular function (EF greater than or equal to 55%). Nevertheless, it was not possible to establish a significant correlation between the extent of fibrosis and the decrease of left ventricular function. Although left ventricular fibrosis could be one of the factors responsible for decreased myocardial function, it is not sufficient to explain the changes of left ventricular function which are rather frequently observed in mitral stenosis.