Since the recently reported relationship between serum fructosamine and IgA concentrations appears to throw doubt on the clinical utility of fructosamine as a measure of hyperglycemic status if IgA concentration is not taken into account, we studied serum immunoglobulin concentrations in 169 diabetics and their relationship with various clinical and analytical parameters. Over 41% of the patients studied had abnormal serum IgA concentrations. Serum IgA concentration was negatively correlated with serum albumin, and among IDDM patients was positively correlated with age (so that the prevalence of abnormal IgA was 57.7% among IDDM patients aged over 30 years). Among NIDDM patients, abnormal IgA concentrations were especially prevalent among those being treated with oral hypoglycemics. Abnormal IgA was also more frequently found in both IDDM and NIDDM patients, who had been under treatment for 10 years or more. Abnormal IgG concentrations were found in 11.8% of the diabetics, and the mean IgM concentration found in the patients was 41.6% lower than in the normoglycemic group. We conclude that abnormal serum IgA concentrations are very common in diabetic patients and that further research should be carried out to verify whether the determination of serum immunoglobulins, IgA in particular, is of clinical use for monitoring diabetes or evaluating its secondary effects.