The authors investigated the effect of engaging in a medical residency program, as a stressful situation, on blood and urine levels of oxidative stress markers. Newly admitted medical residents participated in the study, along with a control group of (nonmedical) students. The authors assessed superoxide dismutase, glutathione peroxidase, catalase, malondialdehyde, micronuclei test, sister chromatid exchange, and 8-hydroxy-2'-deoxyguanosine level. All the biomarkers declined after entrance to the residency program, and the parameters were strongly higher in residents than in the control group. There was no significant relationship between demographic factors and levels of stress biomarkers. The greater extent of oxidative stress may be due to higher tension before entrance to a supposedly critical new position, and the declined levels of biomarkers seen after several months in the program could be attributed to an appropriate adjustment of the residents to the new situation.