The main effect of simvastatin is the decrease of serum cholesterol due to the reduction of LDL. A decrease of serum triglycerides and an increase of HDL-C are commonly observed during the treatment. The reduction of triglycerides is accounted for by the increased catabolism of apo B-containing lipoproteins whereas the mechanisms bringing about the increase of HDL-C are still unknown. We treated 318 patients with primary hyperlipidemia (227 with phenotype IIa and 91 with phenotype IIb) with simvastatin 10 mg a day and after 6 weeks we found a mean 3% increase in HDL-C. HDL-C increased only in about half of the patients and the patients in whom HDL-C increased had baseline higher serum triglycerides and had a greater hypotriglyceridemic response than patients in whom HDL-C did not increase. Accordingly, HDL-C increased in type IIb patients who experienced a greater change in triglycerides than type IIa patients, in whom HDL-C did not increase significantly. Apo A-I levels did not change and apo A-I/HDL-C ratio significantly decreased. At a daily dose of 40 mg, administered to 51 treatment-resistant patients, simvastatin produced a marginally greater decrease in serum cholesterol and LDL-C, but not in serum triglycerides and HDL-C, than at the daily dose of 10 mg. An increase in HDL-C was associated with a reduction in serum triglycerides. The decrease in apo A-I/HDL-C ratio suggests that the increase in HDL-C after simvastatin must be regarded as an enrichment of the cholesterol core of HDL particles. The effect is likely to be due to the decrease of the serum concentration of VLDL bringing about a reduction of cholesterol transfer from apo A-I to apo B-containing lipoproteins.