A quantitative method is described for determination of 4-heptanone and its precursor in urine. The method involves a single-step extraction of urine with cyclohexane and the analysis of 1 mul of the extract by gas chromatography-mass fragmentography. The detection limit is 8-10(-12)g, corresponding to 2 mug per 1000 ml of urine. The method is linear in the concentration range of 7.6-10(-9)-1.5-10(-5)g/ml and the relative standard deviations for the individual values are between 1.2 and 10%. Approximately 40 samples can be analyzed per day or 1 sample in 7-8 min. By varying the experimental conditions, the method is applicable to other ketones.