This paper presents the results of the characterization of 19 industrial customer's measurements of harmonics in their distribution system of medium voltage level, jointly with a software application to carry out a probabilistic evaluation of levels of distortion of currents and harmonic voltages on their systems. The developed program is based on Monte Carlo's simulation technique and it was tested on a distribution feeder circuit. The most important results of this work are the following: A dominant behavior of the harmonic order 3, 5, 7 and 11 was found both in current and in voltage; correlation factors for the magnitudes of the harmonic currents for the three phases is nearly 1; there is a significant presence of multimodal probability density functions in all the analyzed data; with more realistic simulation scenarios the average of Total Harmonic Distortion THD v and THD i indicators are reduced, but standard deviations are increased. ©2008 IEEE.