The objective of this study was to investigate serum adenosine deaminase (ADA) activity as a marker of T lymphocyte activation and parameters of oxidative stress and antioxidant defence in hyperemesis gravidarum (HG). Serum ADA activity, malondialdehyde (MDA), catalase (CAT) and glutathione peroxidase (GP(x)) levels were investigated in 40 pregnant women with the HG and 40 with healthy pregnancies, in a descriptive study. Although serum ADA and CAT were measured to be higher in HG group, the difference was not significant. Serum MDA and GP(x) levels were significantly elevated in women with HG when compared with those without HG. The significance of changes in lipid peroxidation and T-cell activation in the pathogenesis of HG and whether this is a cause or a compensatory reaction to HG requires further investigations with larger multicentre trials.