Our objective was (1) to evaluate total oxidant status (TOS) and total antioxidant status (TAS) in women with nausea and vomiting of pregnancy (NVP) compared with healthy pregnant controls, and (2) to explore any possible relationship between oxidative stress and clinical severity of the disease. Fifty-three women with NVP and 35 healthy pregnant women were enrolled in the study. Of these 53 women, 25 women were classified as mild and 28 women were classified with moderate/severe NVP. The severity of the disease was assessed by Rhodes' index. Plasma levels of TOS and TAS were determined by using automated methods. TOS was significantly higher and TAS was significantly lower in women with NVP ( P < 0.0001, for both). The moderate/severe NVP group had higher levels of TOS and lower levels of TAS than the women with mild NVP ( P < 0.0001, for both). Moreover, Rhodes' index showed positive correlation with TOS (r = 0.563; P < 0.0001) and negative correlation with TAS (r = -0.595; P < 0.0001). These findings indicate that oxidative stress is increased in NVP. There was a strong relationship between oxidative stress and the clinical severity of the disease. Therefore, we suggest that the levels of TOS and TAS can be used as additional markers in the diagnosis and clinical severity of NVP.