In this study we investigated the agewise distributions of serum IgA concentrations in 1251 type 1 and 2224 type 2 diabetic patients, and the association between serum IgA concentration and diabetic complications (retinopathy, neuropathy, nephropathy, macroangiopathy, and hypertension). The IgA concentrations of all groups of diabetic patients were significantly higher than those of the corresponding subgroups of 943 control subjects, except for type 1 patients >60 years of age. High IgA concentrations were found in 23.1% of the whole diabetic group. The prevalence of high IgA was significantly greater in males than in females among type 1 patients (24.4% vs 18%). In conclusion, an increase in circulating IgA concentrations is a generalized phenomenon among diabetic patients; IgA concentrations above the reference range are more common among male than female diabetics; and diabetic complications are associated with a significant increase in serum IgA concentration.