The straight chain aliphatic hydrocarbons ethane and pentane have been advocated as noninvasive markers of free-radical induced lipid peroxidation in humans. In in vitro studies, the evolution of ethane and pentane as end products of n-3 and n-6 polyunsaturated fatty acids, respectively, correlates very well with other markers of lipid peroxidation and even seems to be the most sensitive test available. In laboratory animals the use of both hydrocarbons as in vivo markers of lipid peroxidation has been validated extensively. Although there are other possible sources of hydrocarbons in the body, such as protein oxidation and colonic bacterial metabolism, these apparently are of limited importance and do not interfere with the interpretation of the hydrocarbon breath test. The production of hydrocarbons relative to that of other end products of lipid peroxidation depends on variables that are difficult to control, such as the local availability of iron(II) ions and dioxygen. In addition, hydrocarbons are metabolized in the body, which especially influences the excretion of pentane. Because of the extremely low concentrations of ethane and pentane in human breath, which often are not significantly higher than those in ambient air, the hydrocarbon breath test requires a flawless technique regarding such factors as: (1) the preparation of the subject with hydrocarbon-free air to wash out ambient air hydrocarbons from the lungs, (2) the avoidance of ambient air contamination of the breath sample by using appropriate materials for sampling and storing, and (3) the procedures used to concentrate and filter the samples prior to gas chromatographic determination. For the gas chromatographic separation of hydrocarbons, open tubular capillary columns are preferred because of their high resolution capacity. Only in those settings where expired hydrocarbon levels are substantially higher than ambient air levels might washout prove to be unnecessary, at least in adults. Although many investigators have concentrated on one marker, it seems preferable to measure both ethane and pentane concurrently. The results of the hydrocarbon breath test are not influenced by prior food consumption, but both vitamin E and beta-carotene supplementation decrease hydrocarbon excretion. Nevertheless, the long-term use of a diet high in polyunsaturated fatty acids, such as in parenteral nutrition regimens, may result in increased hydrocarbon exhalation. Hydrocarbon excretion slightly increases with increasing age. Short-term increases follow physical and intellectual stress and exposure to hyperbaric dioxygen.(ABSTRACT TRUNCATED AT 400 WORDS)