The term hypertrophic cardiomyopathy with obstruction encompasses a wide range of clinico-pathological conditions. The mildest forms have localised septal hypertrophy and obstruction only during pharmacodynamic stimulation. The more severe forms have major wall hypertrophy and are obstructive under basal conditions. Mitral systolic anterior motion (SAM) recorded at echocardiography is generally attributed to obstruction. However, the construction of this image by subvalvular structures and the relationship between the obstruction and anatomical deformation led us to study left ventricular haemodynamics with respect to the presence or absence of SAM under basal conditions. Thirty one cases of hypertrophic obstructive cardiomyopathy were divided into 2 groups: -- Group 1 without basal SAM (11 cases); -- Group 2 with SAM under basal conditions (20 cases). Under basal conditions there was no significant difference in LVEDP or ventricular volume between the two groups. An intraventricular pressure gradient was commoner in Group 2 (65% compared to 27%) as was mitral incompetence (53% compared to 27% in the 30 patients undergoing selective left ventriculography). Left ventriculography in the right anterior oblique plane distinguished two types of LV deformation: systolic biloculation of the chamber and systolic apical obliteration. The second form was mainly observed in Group 2. The effect of isoproterenol on LVEDP was studied in 9 cases in Group 1 and 13 cases in Group 2: LVEDP decreased from 14 +/- 6 mmHg to 8 +/- 6 mmHg in Group 1, and increased from 14.5 +/- 6 to 23.5 +/- 6.5 mmHg in Group 2.(ABSTRACT TRUNCATED AT 250 WORDS)