Isoprene is a normal constituent of human breath and may be derived from the cholesterol synthetic pathway. Acute and chronic lovastatin and a cholesterol-supplemented diet were used to determine whether a mechanistic link exists between isoprene and cholesterol biosynthesis in vivo in humans. The acute effects of lovastatin, a competitive inhibitor of the rate-limiting step of cholesterol biosynthesis, on breath isoprene excretion was determined by administering a single 20, 40 or 80 mg dose of this drug to five healthy male subjects at 8 p.m. and measuring their breath isoprene levels every 4 h for one 24 h cycle before and after treatment. When compared to the baseline cycle, all three doses of lovastatin significantly reduced breath isoprene levels at 6 and 10 h post-drug treatment. Chronic lovastatin therapy (40 mg b.i.d. for 6 wk) reduced 6 a.m. breath isoprene levels (time of maximum baseline value) by 27 ± 9% (SEM) and cholesterol synthesis measured in freshly isolated mononuclear leukocytes (ML) by 12 ± 6%. A cholesterol-supplemented diet (1070 mg, total) ingested for 6 wk reduced breath isoprene excretion and ML sterol synthesis by 16 ± 5 and 19 ± 4%, respectively. The parallel decreases in isoprene excretion and cholesterol synthesis caused by these pharmacologic and dietary means suggest that breath isoprene is derived from the cholesterol synthesis pathway.