The objective of this thesis was to determine urinary levels of a biomarker of oxidative DNA damage (8-hydroxydeoxyguanosine, 8-OHdG) in subjects following omnivorous, lacto-ovo vegetarian, and vegan diets. Morning urine samples were analyzed using a solid-phase enzyme-linked immunosorbent assay (ELISA), and the obtained biomarker values were adjusted based on the analyzed urinary creatinine levels. Participants completed a questionnaire with basic demographic and anthropometric data and a three-day food diary (two weekdays and one weekend day). Using a nutrient calculation program on the basis of food intake, energy and nutrient intakes were determined, and the correlation between participants' characteristics and dietary habits with 8-OHdG levels was examined. Female participants on an omnivorous diet had the highest energy intake (1998 ± 293 kcal/day), while vegans had the lowest (1411 ± 280 kcal/day). Despite their low energy intake, vegans the greatest intake of dietary fiber (27.6 ± 11.1 g/day). However, even though they were the most physically active and had the fewest smokers, vegans had the highest average 8-OHdG levels (334.21 ± 175.5 ng/mg creatinine). A statistically significant difference in 8-OHdG levels was found between the vegan and omnivorous groups (p = 0.013). The strongest negative correlation with 8-OHdG was determined for vitamin B12 intake (r = -0.553; p = 0.002). The results suggest that the overall quality of diet may influence oxidative stress, rather than individual nutrients or food groups.