On the basis of 865 registered cases of adverse effects after mass BCG vaccinations in Poland the frequency of these effects has been estimated at 0.2/1000 i.e. 2 cases per 10,000 vaccinated. Among the 865 such cases there were 465 cases (53.8%) of local adverse effects and 365 (43.6%) of lymphadenitis. Within the latter there were 0.04/1000 (i.e. 4 cases per 100,000 vaccinated) of cases with suppurative lymphadenitis. They were found mainly in newborns. No cases of disseminated adverse effects were found. Main errors in the diagnosis and follow-up of adverse effects were lack of data on the diameter of local changes and size of lymphnodes, as well as attempts to remove the lymphnodes. From the above data it can be concluded that BCG vaccinations very rarely produces adverse reaction and therefore may be considered as safe method of tuberculosis prevention.