Purpose: The purpose of this study was to ascertain whether radiation adaptive response could be induced by Tc-99m-methylene diphosphonate (Tc-99m-MDP) in peripheral lymphocytes of patients undergoing bone scintigraphy. Materials and Methods: Lymphocytes from 22 patients (6 males, 16 females, mean age 50¡¾14 years) were collected before and after bone scintigraphy using 740 MBq Tc-99m-MDP. Lymphocytes from 10 controls (6 males, 4 females, mean age 43¡¾7 years) were also collected. They were exposed challenge dose of 2 Gy gamma rays using a cell irradiator Number of ring-form and dicentric chromosomal per 600 cells (chromosomal aberrations) was counted under the light microscope. Results: Chromosomal aberrations in patients before bone scintigraphy (385.1¡¾30.5) was not different from that of controls (367.8¡¾36.6). However, chromosomal aberrations in patients after bone scintigraphy was significantly decreased 192.6¡¾22.1 (p=0.0001). Conclusion: Low dose gamma-irradiation by Tc-99m-MDP used for bone scintigraphy induces a cytogenetic adaptive response in peripheral lymphocytes.