Our work is focused to develop sensitive optical diagnostic methods for the detection of Volatile Organic Compounds in the breath. For the breath measurements, we have built a cavity ring-down spectrometer, operated in the UV region. The system was tested and calibrated using a KinTek automated permeation tube system with nitrogen as a carrier gas, as well as self- prepared acetone mixtures in air with known concentrations. In this paper, we present results of the measurements of acetone concentration in the breath for the test group of randomly selected 40 people in the age group over 50. We observed the values of acetone concentration in the breath in the range from 0,1 – 80 ppm, with a median 7,1 ppm. The experiments showed that the limits of detection for acetone with the cavity ring-down system are several tens of ppb.