Capillary electrophoresis is a fast, automated and highly sensitive technique for the detection of monoclonal components in serum that is being increasingly introduced in clinical laboratories. Nevertheless, false negative and positive results have been reported, and thus the electrophoretic pattern may be difficult to interpret. False positive results are chiefly due to exogenous substances other than proteins present in serum, which absorb at UV wavelengths and can produce an abnormal spike. False negative results are mainly due to very low concentrations of monoclonal components. However, high concentration monoclonal components may not be correctly detected due to the physicochemical properties of the paraproteins, which may cause anomalous separation. In this work, we study three cases with high concentration monoclonal components of the IgA and IgM classes. They were initially detected on cellulose acetate electrophoresis, but the capillary electrophoresis was not able to correctly detect them.