Mixed hyperlipidemia is characterized by both elevated total cholesterol and triglycerides. It is estimated to account for 10% to 20% of patients with dyslipidemia. This study assessed the lipid-altering efficacy and tolerability of simvastatin 40 and 80 mg/day as monotherapy. One hundred thirty patients (62 women [48%], 24 [16%] with type 2 diabetes mellitus, mean age 53 years) with mixed hyperlipidemia (baseline low-density lipoprotein [LDL] cholesterol 156 mg/dl [mean], and triglycerides 391 mg/dl [median) were randomized in a multicenter, double-masked, placebo-controlled, 3-period, 22-week, balanced crossover study, and received placebo, and simvastatin 40 and 80 mg/day each for 6 weeks. Compared with placebo, simvastatin produced significant (p <0.01) and dose-dependent changes in all lipid and lipoprotein parameters (LDL cholesterol 2.1%, -28.9%, and -35.5%; triglycerides -3.5%, -27.8%, and -33.0%; high-density lipoprotein cholesterol 3.3%, 13.1%, and 15. 7%; apolipoprotein B 3.8%, -23.1%, and -30.6%; and apolipoprotein A-I 4.0%, 8.2%, and 10.5% with placebo, and simvastatin 40 and 80 mg/day, respectively). The changes were consistent in patients with diabetes mellitus. One patient taking simvastatin 80 mg/day had an asymptomatic and reversible increase in hepatic transaminases 3 times above the upper limit of normal. Simvastatin 40 and 80 mg/day is effective in patients with mixed hyperlipidemia across the entire lipid and lipoprotein profile. The reductions in LDL cholesterol and triglycerides are large, significant, and dose dependent. The increase in high-density lipoprotein cholesterol was greater than that observed in patients with hypercholesterolemia, and appears dose dependent.