In this study, the characterization of insulin (auto)antibodies has been described, mainly in terms of concentration (q), affinity (Ka) and Ig (sub)isotypes by Surface Plasmon Resonance (SPR) in two particular clinical cases of individuals with severe episodes of impaired glycemia. Subject 1 suffers from brittle diabetes associated with circulating insulin antibodies (IA) due to insulin treatment. Subject 2 has insulin autoantibodies (IAA) associated with hypoglycemia in spite of not being diabetic and not having ever received exogenous insulin therapy. After conventional screening for IA/IAA by radioligand binding assay (RBA), we further characterized IA/IAA in sera of both patients in terms of concentration (q), affinity (Ka) and Ig (sub)isotypes by means of SPR technology. In both cases, q values were higher and Ka values were lower than those obtained in type 1 diabetic patients, suggesting that IA/IAA:insulin immunocomplexes could be responsible for the uncontrolled glycemia. Moreover, subject 1 had a predominat IgG1 response and subject 2 had an IgG3 response. In conclusion, SPR technology is useful for the complete characterization of IA/IAA which can be used in special cases where the simple positive/negative determination is not enough to achieve a detailed description of the disease fisiopathology.