Thiols are antioxidant agents which belong to mercaptan group consisting of sulfur and carbon containing sulfhydryl (-SH). Thiol levels, in the case of elevated oxidative stress, are lowered by their use in neutralization of reactive oxygen molecules. It was aimed to investigate thiol and disulfide blood levels or thiol/disulfide ratio as a marker of total oxidant status which plays a role in the pathogenesis of inflammatory diseases that may cause respiratory failure. The study included 98 patients (58 males, 40 females) who have partial oxygen pressure (PaO2)<60 mmHg or partial carbon dioxide pressure (PaCO2)>45 mmHg in arterial blood in intensive care unit and 98 healthy volunteers. Total thiol and native thiol levels were measured by spectrophotometric method.