A retrospective analysis of the adverse reactions reported between 1979 and 1991, in the 139,000 children under six years of age vaccinated in Sweden with the Danish BCG vaccine, strain Copenhagen 1331, showed an incidence of 1.9 per 1000 vaccinated children. Regional lymphoglandular swellings and/or abscesses were most commonly reported in 1.4 per 1000. Serious, disseminated, BCG infections developed in four infants vaccinated neonatally. Three of the infants suffered from severe, combined, immunodeficiency syndrome, undiagnosed at the time of vaccination. The incidence of severe, combined, immunodeficiency syndrome was higher in the BCG-vaccinated population (4 per 100,000 infants vaccinated within a year of their births), compared with all newborns in Sweden (1 per 100,000). The mean age at the onset of symptoms was 2.4 months for the seven non-BCG-vaccinated infants versus 1.3 months for the four BCG-vaccinated ones, while the immunodeficiency syndrome was diagnosed at an average age of 7.6 months in those who were not vaccinated versus 5.3 months in those BCG-vaccinated. It is recommended that the selective BCG vaccination of infants at high risk of exposure to tuberculosis should be postponed to six months of age to reduce the risk of inoculating infants suffering from immunodeficiency syndromes.