Mycoplasma pneumoniae is a common cause of respiratory tract infections in humans. The aim of the present study was to analyse the seroepidemiology of M. pneumoniae infections in Iceland during a 10-y period. A retrospective analysis of all serological diagnosis of M. pneumoniae infections at the Department of Medical Virology, National University Hospital in Reykjavík was performed. A total of 13,201 test results from 1987 to 1996 were reviewed and altogether 762 patients were found to have raised M. pneumoniae antibody titres, using a conventional complement fixation assay. Infections were most common amongst young people (< or=16 y) but a second peak in incidence was observed around the age of 35 y. Significant annual (p < 0.0001) and seasonal variations (p=0.0003) were observed; M. pneumoniae infections were most common during the winter period. Three major outbreaks with intervals of three to five years were observed during the observation period. Patients diagnosed during these outbreaks had higher M. pneumoniae titres than those found when infections were less frequent (p=0.0017). Furthermore, the middle aged and elderly patients (> 50 y) had significantly lower M. pneumoniae titres than younger patients (p=0.0014). The findings of this study show that M. pneumoniae infections have definite annual and seasonal variations and also confirm previous studies showing community outbreaks of M. pneumoniae infections every 3-5 y.