In a study conducted in Japan, the authors used urinary 8-hydroxydeoxyguanosine (8-OHdG) to study the effects of high-intensity and low-intensity sunlight on oxidative damage to deoxyribonucleic acid (DNA) in patients who had systemic lupus erythematosus (SLE). During late May through early September (i.e., a period of high-intensity sunlight), the mean urinary 8-OHdG level in SLE patients was significantly higher than in controls (31.0 +/- 20.6 [standard deviation] ng/mg vs. 15.4 +/- 7.2 ng/mg, respectively [p < .05]). During late November through early March (i.e., low-intensity sunlight season), however, no significant differences were noted (15.4 +/- 5.5 ng/mg vs. 16.3 +/- 4.6 ng/mg, respectively). The mean urinary 8-OHdG level in SLE patients during the period of high-intensity sunlight was significantly higher than during the period of low-intensity sunlight (21.3 +/- 20.6 ng/mg vs. 12.6 +/- 6.7 ng/mg, respectively; p < .01), although no such seasonal changes were observed among controls (16.2 +/- 8.0 ng/mg vs. 15.7 +/- 5.1 ng/mg, respectively). The effect of sunlight intensity (i.e., season) may require consideration when oxidative DNA damage occurs in individuals who have SLE.