Pentane and isoprene concentrations were analyzed in single end-expiratory breath samples using gas chromatography. Breath analysis was performed in 15 patients with acute myocardial infarction, 15 patients with stable angina, and 15 healthy control subjects. The two patient groups were well matched for age, sex, smoking habits, hypertension and serum cholesterol levels. There was no significant difference in breath pentane concentration in the acute myocardial infarction group (0.29 + 0.03 nmol/l) (mean + SEM) compared to the group with stable angina (0.31±0.03 nmol/l) or the control group (0.36±0.04 nmol/l). However, breath isoprene concentration was higher (p<0.01) in the acute myocardial infarction group (11.4±1.2 nmol/l), compared to both the stable angina group (7.7±0.5 nmol/l) and the control group (7.1±1.0 nmol/l). There was no difference in either the pentane or isoprene concentrations between the control group and the group with stable angina. Since pentane is thought to be an index of lipid peroxidation, the results do not support the presence of enhanced lipid peroxidation in acute myocardial infarction in the absence of thrombolytic therapy or primary angioplasty. The mechanism responsible for isoprene elevation in acute myocardial infarction is unknown.