The "odor of decaying apples" on the breath of patients with severe diabetes was first r orded in 1798 by John Rollo in his classic monograph "Cases of the Diabetes Mellitus".1 In 1857 Petters identified the odor producing substance as acetone2 and in the Bradshawe lecture to the Royal College of Physicians of London in 1886 Dreschfeld stated that the odor of acetone on the breath was a characteristic feature of diabetic coma.3 4 Quantitative methods for measuring acetone were developed in Germany in 1897 by Geelmuyden5 and Nebelthau6 and by 1898 Muller had described a technique suitable for measuring acetone in the breath of patients with diabetes.7 Significant methodological advances were made by Scott-Wilson,8 Marriott9 and by Folin and Denisl° 11 and in 1920 Hubbard12 and Widmark13 successfully measured the concentration of acetone in the breath of normal humans. These chemical methods were insensitive and required tedious procedures for concentrating the sample prior to analysis. Nevertheless mar of these early papers made substantial contributions to our unders.anding of acetone excretion in human breath. 14,15 In 1964 gas chromatography with flame ionization detection was applied to breath acetone measurements by Leveyt6 and by Stewart and Boettner.'7 Breath could be analyzed directly and the rapidity, specificity, sensitivity and economy of the test meant that clinical application was no longer limited by technical or practical difficulties. Since 1964 several groups of investigators have applied breath acetone measurements to clinical problems"'25 but factors such as age, weight, sex, diet, exercise, stress, and diseases other than diabetes that might alter the breath acetone concentration have not been systematically studied. Furthermore standardized methodology has not been employed and the results have not been reported in a comprehensive form. This has made comparative interpretation of new data difficult and has