Hyperemesis gravidarum (HEG) is a severe form of nausea and vomiting with unknown etiology. Recent studies have suggested that there might be an etiologic role for HELICOBACTER PYLORI (HP) in HEG. HP is a Gram-negative bacterium that colonizes the gastric mucosa, increases the generation of reactive oxygen species (ROS), and decreases plasma antioxidants such as ascorbic acid. The aim of this study was to investigate plasma ROS activities and antioxidant status, and their relationship with HP infection, in HEG womens. Twenty-five HEG women, 20 gestational age- and gravida-matched healthy pregnant controls, and 15 nonpregnant women were examined for specific immunoglobulin G (IgG) against HP, serum malondialdehyde (MDA) as a measure of lipid peroxidation, and activity of antioxidant enzymes superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and catalase (CT). Compared with controls, the seropositivity of HP for IgG was significantly higher ( P < 0.01) in pregnancies with HEG. When three study groups (including both HP + and HP- patients) were compared with each other with respect to the study parameters, MDA concentration increased significantly in pregnant women when compared with control group (nonpregnant women; P < 0.01), as well as in HEG compared with healthy pregnant women ( P < 0.01). The SOD, GSH-Px, and CT activity were increased in the group of normal pregnant women versus the nonpregnant group ( P < 0.01), but decreased significantly in the group of HEG pregnant women ( P < 0.01). Serum MDA levels and SOD, GSH-Px, and CT activity were not affected by the seropositivity of HP for IgG in either group ( P > 0.05). The results of the present study suggest that HEG is an oxidative stress condition, as reflected by the increased ROS activity and decreased antioxidant status, regardless of HP infection.