BACKGROUND: Mycoplasma pneumoniae is the most frequent cause of respiratory tract infections in school-aged children and adolescents. M. pneumoniae infection has variable clinical manifestations and is resistant to beta-lactam antibiotics, making correct diagnosis important. We evaluated the newly introduced Chorus M. pneumoniae IgM (DIESSE Diagnostica, Italy) assay for early diagnosis of M. pneumoniae infection. METHODS: The Chorus M. pneumoniae IgM and particle agglutination (PA) (Fujirebio, Japan) assays were tested on 75 serum specimens from 52 hospitalized children at a tertiary-care hospital between September 2011 and November 2011. A positive PA result was defined as an antibody titer of > or =1:40. The concordance of the Chorus M. pneumoniae IgM and PA results and the correlation of the Chorus M. pneumoniae IgM Index with the PA titer were analyzed. Furthermore, the Chorus M. pneumoniae IgM and PA results (PA-patient positive/negative) based on the clinical cutoff of the PA assay were compared in acute-phase specimens. RESULTS: The concordance rate of the Chorus M. pneumoniae IgM and PA results was 90.7% (kappa value= 0.5), and the Chorus M. pneumoniae IgM Index and PA titer correlated well (Spearman's correlation coefficient, 0.872, P < 0.001). However, 82.6% (19/23) of patients who were negative for M. pneumoniae by PA using the clinical cut-off were Chorus M. pneumoniae IgM-positive. CONCLUSIONS: The Chorus M. pneumoniae IgM assay is convenient and gives objective results. However, to make Chorus suitable for routine laboratory use, additional validation studies are required to determine the criteria for use in convalescent specimens.