The purpose of this study was to determine if immune mechanisms in GAD positive patients' contribute to the pathogenesis of a specific sub-type of Type 2 diabetes. GAD positive (n=8) and GAD negative (n=8) subjects diagnosed with Type 2 diabetes were matched for age, gender, body mass index, duration of diabetes and glycaemic control. All subjects underwent an insulin-modified frequently sampled intravenous glucose tolerance test to measure insulin sensitivity and insulin secretory function with minimal model analysis. In addition, BRIN-BD11 clonal beta-cells were supplemented with patients' sera to determine basal and alanine-stimulated insulin secretion and terminal complement complex (TCC) formation. Both groups were severely insulin resistant (0.56+/-0.17 vs. 0.99+/-0.3310(-4)min(-1)/(microUml(-1)) for GADneg and GADpos, respectively) but the GAD negative subjects had a higher basal (87+/-11 vs. 58+/-14pmoll(-1), p<0.05) and glucose-stimulated insulin secretion (DeltaAUCins 0.96+/-0.12 vs. 0.60+/-0.12pmol/(l(-1)min), p<0.05). In vivo measures of insulin secretion were negatively correlated with TCC formation, independent of antibody status. In conclusion, GAD positive subjects initially diagnosed with Type 2 diabetes are unable to compensate for insulin resistance due to more pronounced beta-cell impairment. TCC formation may be partly responsible for the insulin secretory dysfunction associated with this specific sub-type of Type 2 diabetes.