Hypertrophic cardiomyopathy (HCM) is manifested by severe thickening of the left ventricle with significant diastolic dysfunction. Previous observations on the improvement in diastolic function and left ventricular wall thickness through the therapeutic administration of coenzyme Q10 (CoQ10) in patients with hypertensive heart disease prompted the investigation of its utility in HCM. Seven patients with HCM, six non-obstructive and one obstructive, were treated with an average of 200 mg/day of CoQ10 with mean treatment whole blood CoQ10 level of 2.9 micrograms/ml. Echocardiograms were obtained in all seven patients at baseline and again 3 or more months post-treatment. All patients noted improvement in symptoms of fatigue and dyspnea with no side effects noted. The mean interventricular septal thickness improved significantly from 1.51 +/- 0.17 cm to 1.14 +/- 0.13 cm, a 24% reduction (P < 0.002). The mean posterior wall thickness improved significantly from 1.37 +/- 0.13 cm to 1.01 +/- 0.15 cm, a 26% reduction (P < 0.005). Mitral valve inflow slope by pulsed wave Doppler (EF slope) showed a non-significant trend towards improvement, 1.55 +/- 0.49 m/sec2 to 2.58 +/- 1.18 m/sec2 (P < 0.08). The one patient with subaortic obstruction showed an improvement in resting pressure gradient after CoQ10 treatment (70 mmHg to 30 mmHg).