In the recent years the number of commercially available immunoassays for the detection of human cytomegalovirus (HCMV)-specific immunoglobulin M (IgM) antibodies has rapidly increased. The aim of the present study was to evaluate five commercial immunoassays for the serological diagnosis of HCMV-infection. These methods, namely the IMx CMV IgM assay, the AxSYM CMV IgM assay (both Abbott), the Gull CMV IgM, the CMV-IgM-ELA test PCS Medac and the Biotest Anti-HCMV recombinant IgM ELISA, were compared for their diagnostic effectiveness and interference with substances eventually producing cross-reactions with HCMV-IgM (Epstein-Barr-virus (EBV)-IgM, rheumatoid factor (RF)). In addition, repeated measurements on samples from kidney and heart transplant recipients with active HCMV infection were examined to compare the temporal development of the HCMV-IgM measured with the five assay systems. Since there is no commercially available gold standard, it was assumed that the true classification, of whether the patient sample is HCMV-IgM positive or negative, was unknown. Hence sensitivity and specificity were assessed based on a maximum likelihood approach using a "latent class" model. The cross-reactions were quantified by a Bayesian statistical model using prior information for the expected prevalences in the EBV-IgM and rheumatoid factor sample groups. The results of the study demonstrated that there are great differences in sensitivity and specificity as well as in cross-reactions with EBV-IgM and RF between the tested ELISAs.