Background & Objective: Ultrasound waves are extensively used for therapeutic application in physiotherapy. The therapeutic effects of ultrasound often result from the increasing temperature in the exposed tissues, although the nonthermal effects, such as cavitation have an important therapeutic role, the amount of heating depends upon the intensity and frequency of ultrasound. This study was carried out to evaluate the effect of alterations in intensity and frequency of therapeutic ultrasound on chromosome aberrations in human peripheral blood lymphocytes.

  Methods: This study which was a longitudinal and semi experimental one, was carried out in vivo condition as pre and post treatment. Twenty individuals were selected nonrandomly out of the patients referring to the physiotherapy unit. The studied samples were provided into four stages of before the treatment, the end of the fifth session, the end of the tenth session and a week after the end of the treatment, respectively. The treatment was carried out using ultrasound waves in different intensities and frequencies and the results obtained before the treatment were compared with three stages after the treatment. The results were analyzed by Wilcoxon and Friedman statistical tests.

  Results: The mean of the total number of chromosome aberrations at the first stage before exposure was 0.45 percent that in comparison comparing with the second stage after exposure it reached 1.62 percent (p=0.02), at the third stage to 3.23 percent (p=0.008) and at the fourth stage to 1.59 percent. A significant increase was observed between increase of ultr asound intensity and chromosom al aberrations frequency (p=0.002).

  Conclusion: It seems that exposure to therapeutic ultrasound waves with high intensities leads to an increase in the frequency of chromosome aberrations in the individuals undergoing exposure. However, increasing the frequency of these waves did not have significant effects on chromosome aberrations of the studied specimens.