Many patients with high levels of serum total cholesterol have a concomitant elevation of serum triglyceride levels and thus have mixed hyperlipidemia. In this study, 13 patients with mixed hyperlipidemia were treated with the cholesterol-lowering drug lovastatin to determine its effectiveness. In 9 of these patients, lovastatin therapy used alone was compared with the drug combination of lovastatin and gemfibrozil. In the 13 patients, lovastatin therapy produced a 31% reduction in total cholesterol level and a 32% decrease in triglyceride levels compared with placebo. It lowered very-low-density plus intermediate-density lipoprotein cholesterol levels by 40%, low-density lipoprotein cholesterol levels by 36%, and total apolipoprotein B levels by 28%. Concentrations of high-density lipoprotein cholesterol and apolipoprotein A-I were unchanged, but total cholesterol (and low-density lipoprotein cholesterol)/high-density lipoprotein cholesterol ratios were markedly reduced. Compared with lovastatin alone, lovastatin plus gemfibrozil produced greater decreases in very-low-density plus intermediate-density lipoprotein cholesterol levels and an increase in high-density lipoprotein cholesterol levels, but, in view of the higher risk for severe myopathy with this combination, lovastatin used alone may be adequate therapy for many patients with mixed hyperlipidemia.