Acetone can be present in a person's blood due to several conditions: diabetic or alcoholic ketoacidosis (the two main reasons), fasting, low carbohydrate diet, or the ingestion of acetone and/or isopropanol. It is thus possible that a subject may have acetone in his/her blood, which, being volatile, will be present in the breath along with alcohol. The Intoxilyzer 5000C is widely used to measure the BAC through the analysis of breath samples. This instrument has a built-in acetone correction procedure that is designed to handle the presence of a small amount of acetone in the blood without any error message; at higher concentrations of acetone, an error message may appear. The current study examined the instrument's effectiveness in measuring breath BAC in the presence of different acetone concentrations. Measurements of headspace vapors of alcohol standards and alcohol-acetone mixed standards delivered from simulators were taken with the Intoxilyzer 5000C. The alcohol concentrations in the aqueous solutions used were such that the target value of headspace vapor would be 0, 50, 100, 150, and 200 milligrams of alcohol/100 milliliters of solution. Acetone concentrations in the solutions were 0, 1, 3, 5, 10, 25, 32, 34, 35, 40, 100, and 500 milligrams/100 milliliters. Measurements were repeated 15 times, and the error message ("INTERFERENT" or "INVALID SAMPLE"), if any, was noted in each case. The "INTERFERENT" message consistently appeared above the calibration level of 35 milligrams/100 milliliters of acetone in the solution. Even when the "INTERFERENT" message appeared, the headspace alcohol concentration measured was accurate within plus or minus 10 milligrams/100 milliliters. The study concludes that the Intoxilyzer 5000C produces accurate BAC measurements in the presence of acetone up to 100 milligrams/100 milliliters. 1 table, 4 figures, and 6 references