Gestational diabetes mellitus (GDM) has been described in 1-3% of pregnancies and increases the risk (up to 60-70%) to subsequently developing an overt diabetes (generally of type 2 non insulin-dependent diabetes mellitus (NIDDM)). Several humoral autoimmune phenomena have been described in GDM: islet cell antibodies (ICA) have been found and it was shown that ICA+ patients tend to have a worse glucose tolerance. Recently, autoantibodies against glutamic acid decarboxylase (GAD), were detected in type 1 diabetic sera before or at the onset of the disease; these markers, as well as ICA and insulin antibodies, seem to have a predictive value for the onset of the disease. Aim of our study was to investigate the presence of GAD65 in 83 GDM, 79 NIDDM and 64 pregnant normal women in late gestation. GAD Ab positivity was found (0.035 index as limit) only in GDM and NIDDM (3.6% in GDM, 3.8% in NIDDM, and nothing in control women). These results indicate that GAD positivity in GDM overlaps that of NIDDM, suggesting that the two diabetic populations have the same predisposition to develop a type 1 diabetes mellitus, and likely they share the same disease. Further studies need to clarify whether this prevalence of GAD positivity may unmask type 1 diabetes in both GDM and NIDDM diabetic women.