The changes of mitral valve echo and hemodynamic data [isovolumic relaxation time (IRT)/square root R-R, time constant T, peak positive dP/dt/P, left ventricular enddiastolic pressure (LVEDP) and left ventricular systolic pressure (LVSP] during acute pressure overload produced by aortic root obstruction were analyzed in 13 mongrel dogs under sodium pentbarbital anesthesia (25 mg/kg). IRT/square root R-R, time constant T, positive dP/dt and LVSP were expressed as percent changes to the value (=100%) of pre-pressure overload, LVEDP was expressed by an absolute value as mmHg. In 7 of 13 dogs, an abnormal diastolic monophasic triangular pattern of the mitral valve was observed during acute pressure overload of the left ventricle, and values of five hemodynamic data were compared between cases with or without the triangular pattern. The values of IRT/square root R-R, time constant T, positive dP/dt/P, LVSP amd LVEDP in cases with the triangular pattern became from 200 to 500% (275 +/- 100%), from 175 to 267% (220 +/- 50%), from 55 to 112% (81 +/- 21%), from 129 to 200% (59 +/- 21%) and from 7 to 33 mmHg (16 +/- 9 mmHg), respectively. The values of IRT/square root R-R, time constant T, positive dP/dt/P, LVSP and LVEDP in cases with the non-triangular pattern became from 116 to 155% (133 +/- 17%), from 116 to 154% (136 +/- 16%), from 111 to 186% (62 +/- 34%) and from 9 to 20 mmHg (9 +/- 6 mmHg), respectively. Thus, the values of IRT/square root R-R and time constant T were significantly different between the two groups. The possible explanation for the triangular pattern of the mitral valve seems to be due to impaired active relaxation system of the left ventricle resulting in a markedly delayed opening of the mitral valve. We conclude that early diastolic isovolumic relaxation of the left ventricle is impaired by acute pressure overload, and the echocardiographic diastolic monophasic triangular pattern of the mitral valve reflects this impairment.