Lipid peroxidation has gained increasing interest in recent years as one of the more prominent features of free radical-induced damage in biology. The study of lipid peroxidation might increase our understanding of the etiology and pathophysiology of a great number of diseases. Ethane and pentane are among the numerous end-products of lipid peroxidation and although they represent only a small and possibly variable proportion of the total amount of peroxidized polyunsaturated fatty acids, their determination in head space or exhaled breath enables accurate assessment of oxidative stress both in vitro and in vivo. To date, the number of studies utilizing the hydrocarbon breath test as a marker of lipid peroxidation in humans is small. Technical difficulties are among the main reasons for the limited use of this method. An appropriate washout period, the use of the right materials, the scrupulous avoidance of air contamination, adequate preinjection concentrations of the samples, and a sensitive gas chromatographic technique enable the accurate and reproducible measurement of hydrocarbons in human breath. The hydrocarbon breath test provides a noninvasive and extremely sensitive instrument for the assessment of oxidative stress status in adults as well as in children.