In the Helsinki Heart Study, a randomized five-year, double-blind trial, a 34% reduction in the incidence of coronary heart disease (CHD) was observed in dyslipidemic men treated with gemfibrozil. Averaged over the five years of the trial, gemfibrozil therapy produced, compared with placebo, mean decreases of 10% in serum total cholesterol level, 14% in non-high-density lipoprotein (HDL) cholesterol level, 11% in low-density lipoprotein (LDL) cholesterol level, 35% in triglyceride level, and a mean increase of 11% in HDL cholesterol level from baseline levels measured prior to treatment. While changes in HDL cholesterol level were similar in all Fredrickson types, the effect on concentrations of total cholesterol and LDL cholesterol was largest in type IIA and on LDL minimal in type IV. The reduction of CHD incidence over placebo was largest in type IIB and smallest in type IIA. The lipid changes were dependent on lipid levels prior to treatment and on compliance with the medication regimen. When risk factors for CHD, including age, blood pressure, smoking and drinking habits, baseline lipid levels, and exercise and relative weight, were controlled by applying the Cox proportional hazards model, the changes in serum HDL and LDL cholesterol levels were both statistically significantly associated with the decline in CHD incidence within the gemfibrozil-treated group. The large decrease in serum triglyceride levels had relatively small effect on CHD incidence. Thus, the results of this study, together with earlier observations, suggest that both elevating HDL and lowering LDL cholesterol levels are effective in the primary prevention of CHD.