The clinical profiles in patients with hypertrophic cardiomyopathy who had exercise-induced deterioration in systolic performance of the left ventricle (LV) were investigated using exercise echocardiography. The materials consisted of 32 patients, which who categorized in two groups according to the extent of % shortening fraction of the LV (% SF) at the peak exercise; 21 whose % SF was increased (group I: from 40.9 +/- 7.2% at rest to 44.2 +/- 8.0% at the peak exercise) and 11 whose % SF was decreased (group II: from 40.8 +/- 7.3% to 34.8 +/- 6.9%). There were no significant differences between these two groups as to the resting echocardiographic data or the prevalence of pressure gradient in the LV outflow tract. The frequency of symptoms, such as chest pain and exertional dyspnea, was higher in the group II (73%) than in the group I (38%). The time of exercise tolerance was significantly shorter in group II than in group I (I: 9.2 +/- 1.9 min., II: 7.4 +/- 2.6 min., p less than 0.05). Five patients (45%) in group II and four (19%) in group I developed at least 2 mm ST segment depression during exercise electrocardiography. Twenty-four hour ambulatory ECG monitoring showed a high prevalence of ventricular arrhythmias in group II. Seven (78%) of nine patients in group II and five (28%) of 18 in group I had abnormal 201T1 myocardial scintigrams. Left ventricular ejection fraction was not significantly different between the two groups, but the end-diastolic pressure was higher in group II (19 +/- 6 mmHg) than in group I (15 +/- 4 mmHg). All patients who underwent coronary arteriography had no significant stenosis. Thus, there are significant differences in the clinical features between the two groups of patients who had reciprocal LV responses during exercise. These findings should be considered in the management of patients with hypertrophic cardiomyopathy.