Effect of ionizing radiation on the hypoxanthine-guanine phosphoribosyl-transferase (HPRT) gene in the peripheral lymphocytes of patients with oral cancer was studied. Subjects were randomly separated into five groups: A) healthy subjects without smoking and betel quid chewing habits; B) are healthy non-smokers but with betel quid chewing habit. Patients with oral cancer are all smokers and betel quid users, and are separated into three groups: pre-, during, and six months after radiotherapy (RT), respectively. The HPRT variant frequencies (Vfs) of groups A and B are (8.68 ± 2.1) × 10^(-6) and (9.48 ± 3.73) × 10^(-6), respectively. Notably, Vfs are (10.02 ± 2.68) × 10^(-6), (92.75 ± 17.48) × 10^(-6) and (61.72 ± 16.65) × 10^(-6), respectively, for groups C, D, and E. Both during- and post-RT groups demonstrate a significantly higher HPRT Vf as compared with that of pre-RT (P < 0.01). The effect of gender on HPRT Vf was also found in post-RT group (P < 0.05). In conclusion, oral cancer patients treated with RT resulted in the increase of HPRT gene mutation during and even six months post the treatment.