A method is described for rapid measuring the ethane concentration in exhaled human breath. Ethane is considered as a volatile marker for lipid peroxidation. The breath samples are analyzed in real-time during single exhalations by means of infrared cavity leak-out spectroscopy. This is an ultra-sensitive laser-based method for the analysis of trace gases on the sub-ppb level (parts per billion). We demonstrate that this technique is capable of online quantifying ethane traces in exhaled human breath down to 500 ppt (parts per trillion) with a time resolution better than 800 ms. This study includes what we believe to be the first measured expirograms for trace fractions of ethane. The expirograms were recorded after a controlled inhalation exposure to 1 ppm (parts per million) of ethane. The normalized slope of the alveolar plateau (S_n) was determined, which shows a linear increase over the first breathing cycles and ends in a mean value between 0.21 l^-1 and 0.39 l^-1. The washout process was observed for a time period of 30 min and modelled by a threefold exponential decay function, with decay times ranging from 12 s to 24 s, 341 s to 481 s, and 370 s to 1770 s. Our analyzer provides a promising non-invasive tool for online monitoring of the oxidative stress status.