In four healthy cows an elevation of ketone bodies was induced by reduction of feed intake. Two cows became clearly ketotic while the other two cows showed only slight increases in ketone body concentrations in serum and milk. Acetone concentrations in exhaled breath were measured by gas chromatography combined with mass spectrometry. These values were correlated with concentrations of serum beta-hydroxybutyric acid (r = 0.81) and milk acetoacetate+acetone (r = 0.70). It is concluded that the ketotic state of dairy cows can be detected by analysis of exhaled breath. This offers a potential non-invasive method of determining the metabolic state of dairy cows.