Left ventricular myocardial biopsy was carried out at operation in 11 patients with pure, isolated mitral stenosis after preoperative angiocardiographical assessment of left ventricular function. The biopsy specimens were examined by light and electron microscopy. The diameter of the myocytes was normal (20 +/- 1.6 mu). The changes observed were probably of a degenerative type with anarchy and irregularities in the sarcomeres, modification of the Z bands, as seen in Nemaline myopathies, and changes in the intercalatory discs. Moderate interstitial fibrosis with scanty histiocytes was also observed. A quantitative assessment by two dimensional planimetry showed a significant increase in the interstitial space (37 +/- 5.5%) compared to a control group without fibrosis (22 +/- 1.1%). The angiocardiographical indices of left ventricular function were all decreased. The amplitude of circumferential fibre shortening was reduced: 25 +/- 6% the ejection fraction by 52 +/- 9% and the average speed of circumferential fibre shortening by 1.0 +/- 0.3 circ/s. Only four patients had normal left ventricular function (ejection fraction > 55%). However, it was not possible to establish a significant correlation between the degree of fibrosis and the reduction in left ventricular function. Left ventricular fibrosis may be one of the factors responsible for the reduction of myocardial function, but it does not in itself explain all the changes in left ventricular function observed in mitral stenosis.