Serum coenzyme Q10 (CoQ10: 2-(3,7,11,15,19,23,27,31,35,39-decamethyl-2,6,10,14,18,22,26,30,34 ,38 -tetracontadecaenyl)-5,6-dimethoxy-3-methyl-1,4-benzoquinone, CAS 303-98-0) and cholesterol levels were measured to assess the effect of cholesterol-lowering therapy in patients with non-insulin-dependent diabetes mellitus (NIDDM). Twenty healthy volunteers, 97 NIDDM patients and 2 patients with familial hypercholesterolemia were studied. None had overt heart failure or any other heart disease. Mean serum CoQ10 concentrations were significantly (p < 0.01) lower in diabetic patients with normal serum cholesterol concentrations, either with or without administration of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (HMG-CoA RIs) including simvastatin (normal: 0.91 +/- 0.26 (mean +/- SD) mumol 1(-1); diabetic with HMG-CoA RI: 0.63 +/- 0.19; diabetic without HMG-CoA RI: 0.66 +/- 0.21). CoQ10 concentrations were higher (1.37 +/- 0.48, p < 0.001) in diabetic patients with hypercholesterolemia. Simvastatin or low density lipoprotein apheresis decreased serum CoQ10 concentrations along with decreasing serum cholesterol. Oral CoQ10 supplementation in diabetic patients receiving HMG-CoA RI significantly (p < 0.001) increased serum CoQ10 from 0.81 +/- 0.24 to 1.47 +/- 0.44 mumol 1(-1), without affecting cholesterol levels. It significantly (p < 0.03) decreased cardiothoracic ratios from 51.4 +/- 5.1 to 49.2 +/- 4.7%. In conclusion, serum CoQ10 levels in NIDDM patients are decreased and may be associated with subclinical diabetic cardiomyopathy reversible by CoQ10 supplementation.