SummaryYeast of Malassezia, members of the microbiologic flora of the skin, cause pityriasis versicolor and have also been implicated in the pathogenesis of other superficial dermatoses. The mechanisms by which the yeast cause these dermatoses, however, are not yet clear, and there have been no studies on the interaction between fungi and keratinocytes, especially the effects of fungi on the production of reactive oxygen/nitrogen species by human keratinocytes. This is the first study we aimed to evaluate the role of oxidative/nitrosative stress in affected skin areas in patients with pityriasis versicolor. Sixty pityriasis versicolor patients and 60 healthy subjects as control group were enrolled. Scraping samples were obtained from the lesional skin areas in patients and from the normal skin areas in controls. The distribution of lesions of pityriasis versicolor was mild in all patients. We have isolated two Malassezia species: M. furfur being isolated in 75% of patients, followed by M. globosa (25%). The levels of oxidative/nitrosative stress biomarkers were significantly higher in patients than the controls (P < 0.001). Our results demonstrated that pityriasis versicolor is associated with significant alteration in oxidant/antioxidant balance. It might be secondary consequence of pityriasis versicolor infection or contributing factor in its pathogenesis.