Suppression of cholesterol synthesis by 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors, such as lovastatin, has been shown to inhibit mitogen stimulated proliferation of natural killer (NK) cells and other lymphocytes in vitro. This effect is only partially overcome by provision of exogenous free or lipoprotein cholesterol but is reversed by mevalonate, suggesting that proliferating lymphocytes have a specific requirement for a nonsterol isoprenoid product of mevalonate. The effect of lovastatin (20 mg bid) on a range of immune function parameters was determined in a randomized, placebo-controlled, double-blind ex vivo study in 52 patients with primary hypercholesterolemia. No significant differences (P < 0.05) were found between lovastatin and placebo groups for basal NK or interleukin-2 (IL-2)-induced cell-mediated cytotoxicity, PHA-stimulated lymphocyte proliferation, or relative numbers of T lymphocytes (CD3+), B lymphocytes (CD19+), total NK cells (CD3-, CD16+, CD56+) and CD57+ NK cells or in immunoglobulin levels after 4 or 8 weeks of treatment. In contrast to previous in vitro data, no statistically or clinically significant changes were observed in any parameter of lymphocyte function in patients treated with lovastatin.