A retrospective study was conducted to evaluate the utility of Mycoplasma pneumoniae IgG (quantitative), IgA (quantitative), IgM (qualitative) ELISA kits (Medac Diagnostika, Germany) for the diagnosis of M. pneumoniae pneumonia in children under 16 years of age. This study included a total of 159 serum samples from 113 patients with acute respiratory diseases such as bronchitis, pneumonia, which were classified into three groups according to the results of a particle agglutination (PA) test as a reference method, that is, Group I (Mycoplasma-definite cases): Group I-a (paired 52 samples from 26 cases); a four-fold or greater rise of antibody from an acute phase PA titer of < or = 1:80, Group I-b (paired 12 samples from 6 cases); a four-fold or greater rise of antibody from an acute phase PA titer of > or = 1:160, Group I-c (48 samples from 38 cases); a single high PA titer of > or = 1:640 either or both in acute or convalescent serum, Group II (Mycoplasma-probable cases, 18 samples from 17 cases): a PA titer of 1:160 or 320 was observed either or both in acute or convalescent serum, but the above serological criteria for Group I were not fulfilled, Group III (non-cases, 29 samples from 26 cases): a PA titer of any sample was < or = 1:80. The ELISA tests were performed according to the supplier's recommendations, and the results were classified according to the interpretation provided by the supplier: Early stage of infection (category 1,2), Acute- (3,4,5), Current- (6), Past- (7), and No-infection (8). The day of onset of fever (defined as a body temperature of > or = 37.5 degrees Celsius) was denoted as day 0. As a result from Group I, the category initially observed following the onset of fever was category 8 (triple negative), and the predominance of category 8 was replaced by category 1 (IgM solely positive) after day 4, followed by a shift of predominance to category 4 (IgM and IgG double positive) or 5 (triple positive) after day 10 or later. Specifically, category 1 was rather exclusively observed before day 21 following the onset of fever. These results suggest that category 1, when observed, is a useful marker of acute infection by Mycoplasma pneumoniae in children because it appears early in the acute phase and no longer observed beyond the convalescent phase. On the other hand, significance of detecting IgA antibody, which must be important for adults, was not remarkable in our study. Five samples in group II and 3 samples in group III fell into category 1. Whether or not such cases, in the absence of significant PA titers, can be taken actually as mycoplasmal infection remains to be clear. This study validated the utility of this ELISA methodology in terms of the acute phase diagnosis using a single point serum sample for Mycoplasma pneumoniae infection specifically in children.