Aims/hypothesis. Gestational diabetes mellitus is a heterogeneous disorder characterised by impaired insulin secretion and action. Our aim was to study whether autoimmunity, variations in genes affecting insulin secretion and action, or both, contribute to the development of gestational diabetes and whether the pathogenesis of the disease differs between women with a Scandinavian or Arabian background. Methods. We studied a total of 500 unrelated women with gestational diabetes (400 Scandinavian and 100 Arabian) and 550 unrelated pregnant non-diabetic control women (428 Scandinavian and 122 Arabian) matched for ethnicity. Results. Arabian women with gestational diabetes were 50% more insulin resistant for the same BMI compared with Scandinavian women with the disease (homeostasis model assessment [HOMA-IR]; 3.2±0.3 vs 2.2±0.2, p=0.02). Both Scandinavian (4.2% vs 0.9%, p=0.008) and Arabian (4.6% vs 0.0%, p=0.03) women with gestational diabetes had a higher frequency of GAD antibodies (GAD65Ab) than the matched controls. The frequency of HLA-DQB1 risk genotypes was slightly higher in Scandinavian women