The biosynthetic pathway of the CoQ polyisoprenoid side chain, starting from acetyl-CoA and proceeding through mevalonate and isopentenylpyrophosphate, is the same as that of cholesterol. We performed this study to evaluate whether vastatins (hypocholesterolemic drugs that inhibit HMG-CoA reductase) modify blood levels of ubiquinone. Thirty-four unrelated outpatients with hypercholesterolemia (IIa phenotype) were treated with 20 mg of simvastatin for a 6-month period (group S) or with 20 mg of simvastatin plus 100 mg CoQ10 (group US). The following parameters were evaluated at time 0, 45, 90, 135 and 180 days: total plasma cholesterol (TC), HDL-cholesterol, LDL-cholesterol (LDL-C), triglycerides (TG), apo A1, apo B and CoQ10 in plasma and platelets. In the S group, there was a marked decrease in TC and LDL-C (from 290.3 mg/dl to 228.7 mg/dl for TC and from 228.7 mg/dl to 167.6 mg/dl for LDL-C) and in plasma CoQ10 levels from 1.08 mg/dl to 0.80 mg/dl. In contrast, in the US group we observed a significant increase of CoQ10 in plasma (from 1.20 to 1.48 mg/dl) while the hypocholesterolemic effect was similar to that observed in the S group. Platelet CoQ10 also decreased in the S group (from 104 to 90 ng/mg) and increased in the US group (from 95 to 145 ng/mg). This study demonstrates that simvastatin lowers both LDL-C and apo B plasma levels together with the plasma and platelet levels of CoQ10, and that CoQ10 therapy prevents both plasma and platelet CoQ10 decrease, without affecting the cholesterol lowering effect of simvastatin.