We examined the stabilities of urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) stored at room temperature (25 degrees C) for 24 h or at -80 degrees C for 800 days. Subjects were 19 males and 17 females aged 23-58 yr for the 24-h study, and 9 males and 4 females aged 24-54 yr for the 800-day study. We obtained information on the subjects by questionnaires and interviews. The level of urinary 8-OHdG was measured by HPLC using two-step separations. There were no significant changes of amount of urinary 8-OHdG under either storage conditions. We conclude that urine samples can be stored at 25 degrees C and below for 24 h, when the research purpose includes the determination of urinary 8-OHdG. Urinary 8-OHdG was also stable for over two years when stored at -80 degrees C.