Objectives: To report an epidemic ofMycoplasma pneumoniaein Manitoba and to discuss the limitations of the serodiagnostic tests used.Design: A retrospective analysis of the results of a province-wide serological testing for respiratory infections caused byM pneumoniae,using a complement fixation test and an indirect immunofluorescent antibody test for the detection of immunoglobulin (Ig) M antibodies.Material: From April 1, 1987, to March 31, 1991, 12,804 sera were tested and a serological diagnosis of recentM pneumoniaeinfections were established in 509 (3.97%). From April 1 to September 30, 1991, an additional 2088 persons were tested; the 158 (7.5%) recent cases ofM pneumoniaewere subjected to analysis.Results: Compared with the previous three years, an increase in the number of recent cases ofM pneumoniaewas first noticed in July 1990 which persisted until September 1991. Of 856 single sera tested, 59 (6.8%) were recentM pneumoniaeinfections and 56 (96.1%) of these were positive for IgM antibodies. Of the 616 persons who submitted paired sera, 99 (16%) were recent infections, but only 46 (46.4%) had IgM antibodies. Primary infections (ie, positive for IgM antibodies) were detected in 102 (64.5%) and reinfections (ie, positive complement fixation test only) in the remaining 56 persons with recentM pneumoniaeinfections. Primary infections were detected more frequently in the ‘under 16’ than in the ‘over 16’ year age group (75% versus 55.8% of the recent cases ofM pneumoniaein each age group). Reinfections were more common in the older age group. Of the 158 recent cases ofM pneumoniae,30.3% had a pneumonia; of these, 21 (55.2%) were under the age of 16 years.Discussion:M pneumoniaeis an important cause of morbidity. Serological tests are used for the diagnosis despite their limitations. The detection of IgM antibodies in acute serum establishes a diagnosis of primaryM pneumoniae;however, their absence does not excludeM pneumoniae. Asecond (convalescent) blood test is required to diagnose all primary infections. To diagnose all reinfections, paired sera should be tested by complement fixation.Summary: Manitoba experienced an epidemic ofM pneumoniaein 1990–91. Properly selected serological tests can provide a specific and rapid diagnosis.