This study analyses the numbers of serologically verified Mycoplasma pneumoniae infections in Poland in 1970-1995. The investigations were performed using the complement fixation test (CFT) with sonicated antigen in National Institute of Hygiene in Warsaw and since 1985 in 33 laboratories throughout the country. The result was accepted as positive when antibody titre was 60 or higher, or at least a four-fold increase in titre occurred during the illness. During these studies five epidemics of mycoplasmosis were noted in Poland. They occurred regularly every 5 years during the autumn-winter season in 1970/1971, 1975/1976, 1980/1981 and 1985/1986. The last epidemic, which started in 1991 and culminated in 1992-1993, seems to have inaugurated a change from epidemic to endemic occurrence of M. pneumoniae infection in Poland. At the peak of the epidemic, depending on the region of country, in 20-38% of patients with respiratory tract infection serological confirmation of mycoplasmosis was obtained.