During the years 1970-1993 diagnostic serological tests directed against infection with M. pneumoniae were performed in 89,155 persons, mostly children at the preschool and school age with clinical symptoms of respiratory tract infections. Investigations were performed by application of the complement fixation test with cellular antigen of M. pneumoniae. Positive result was established when antibody titer was 60 or higher, or at least four-fold increase o titer occurred during the illness. During performance of these studies five epidemics of mycoplasmosis were noted in Poland. They occurred during the autumn-winter season in 1970/71, 1975/76, 1980/81, 1985/86 and 1991-1993. At the peak of the epidemic depending of the region of country, in 25-38% of patients with pneumonia serological confirmation of mycoplasmosis was established. In selected group of 251 persons which consisted in 87% of hospitalized patients with pneumonia, serological test was repeated at least twice during 7-10 days confirming mycoplasmal etiology of illness in 48.6% of cases. It was found that performance of serological test only once during first week of clinical symptoms of the disease decreases chances of mycoplasmosis diagnosis by 53%.