Measurement of HDL-cholesterol is important in the risk evaluation of cardiovascular disease. Recently, we encountered a 61-year-old man with a negative result for HDL-cholesterol using the Roche assay. Further analysis revealed that this patient had a monoclonal Ig type IgMkappa, with an electrophoresis-based concentration of 55.8 g/L, which apparently caused the negative result for HDL-cholesterol. Analysis of serum samples from other patients with monoclonal or polyclonal hypergammaglobulinaemia revealed that false negative results in the Roche assay for HDL-cholesterol may occur incidentally. Since negative results are detected easily in the laboratory, such patients do not pose a problem in daily practice. However, it is reasonable to suppose that false low results could also be caused by monoclonal as well as polyclonal Igs. Since such false low results remain undetected, vascular risk estimates consequently are altered and unnecessary treatment might be started.