INTRODUCTION: Diazepam (DZ) belongs to benzodiazepines, a group of drugs used for sedation, for the relief of anxiety, and in the treatment of epilepsy. It has been found that DZ influences cytotoxic activity and diminishes antiviral and antitumor reactions in human natural killer cells in vitro. It has also been demonstrated that DZ causes a significant increase in the frequency of chromosomal aberrations in human lymphocytes in vitro.MATERIALS AND METHODS: In the present research the cytogenetic effects of DZ have been studied in normal human lymphocyte cultures of peripheral blood at 17.6-211.2 microM (final concentrations). Sister chromatid exchanges (SCEs), one of the most sensitive methods reflecting instability in DNA or a deficiency in DNA repair mechanisms, and proliferation rate index (PRI), a valuable indicator for cytostatic activity, have been evaluated.RESULTS: After 72-h incubation, DZ was found to cause a dose-dependent, statistically significant increase of SCE frequency (p < 0.001), followed by an equally significant decrease of PRI (p < 0.001).CONCLUSION: Our results suggest that DZ's administration presents cytogenetic effects in normal human lymphocyte cultures.