Mycoplasma pneumoniae is a common cause of community-acquired respiratory infection in children. An estimated 30% of Mycoplasma pneumoniae infections in those 5–15 years of age result in pneumonia and, overall, Mycoplasma pneumoniae is responsible for 10–40% of all cases of community-acquired pneumonia in children [1, 2]. A specific diagnosis of Mycoplasma pneumoniae infection is important since treatment with routinely used β-lactam antibiotics is ineffective against these organisms which lack a cell wall. Proper treatment of infected patients with macrolide antibiotics shortens the duration of overt infection and decreases the severity of respiratory symptoms [2, 3]. Unfortunately, the prompt and accurate diagnosis of Mycoplasma pneumoniae infection in children is hampered by the lack of standardized, rapid, and specific testing methods. Current methods for the specific diagnosis of Mycoplasma pneumoniae infection include culture, serology, and polymerase chain reaction (PCR). Serology has been the mainstay for the diagnosis ofMycoplasma pneumoniae infections, particularly since culture methods are insensitive, require specialized techniques, and at least 2–3 weeks for results [4]. PCR is a promising diagnostic tool for the identification of Mycoplasma pneumoniae in clinical specimens, but it is available only in specialized laboratories. No PCR kits approved by the Food and Drug Administration of the USA are yet available, and inhouse performance characteristics vary substantially among laboratories [3–5]. Results of several studies evaluating various serologic methods have been published, but an assessment of the actual performance of the tests is hampered by the lack of standardized, comparative methods [6–9]. Older methods have tested for complement fixing antibodies that measure predominantly immunoglobulin (Ig)G and usually peak 1– 3 weeks after the onset of infection. Because complement fixing antibodies can persist for several years after infection, it is necessary to obtain paired acute-phase and convalescent-phase sera to demonstrate seroconversion or a fourfold increase in titer for reliable diagnosis. The need for both specimens typically affords only a retrospective diagnosis of acute infection. Enzyme immunoassay (EIA) and indirect fluorescent antibody test methods may provide a more sensitive and specific diagnosis compared to the complement fixation test [8–10]. The Meridian ImmunoCard Mycoplasma IgM assay (Meridian Bioscience, Cincinnati, OH, USA) is a membrane-based, qualitative EIA in which patient sera is allowed to react with Mycoplasma pneumoniae antigen extracts impregnated on filter paper. It requires minimal hands-on time, no additional equipment, and includes an internal control. In the study reported here, the performance of the Meridian ImmunoCard Mycoplasma test was compared to two conventional microtiter plate EIAs for the detection of IgM antibodies to Mycoplasma pneumoniae using acute-phase serum samples from 145 pediatric patients. The average age of the patients was 7.1 years (range, 8 months–17 years) and 46% were female. Initially, all samples were assayed using the ImmunoCard Mycoplasma IgM assay (Meridian Bioscience) according to the manufacturer’s instructions. Samples were stored at −30°C, thawed once, and retrospectively analyzed using the ImmunoWELL Mycoplasma pneumoniae IgM EIA (GenBio, San Diego, CA, USA) as recommended by the manufacturer. A subset of samples with discrepant results between ImmunoCard and ImmunoWELL were tested by a third EIA method, the Zeus J. J. Dunn (*) . J. Evans Department of Pathology, Cook Children’s Medical Center, Fort Worth, TX, 76104, USA e-mail: jdunn@cookchildrens.org Tel.: +1-682-885-6475 Fax: +1-682-885-6111