The aim of this study was to investigate the teratogenicity of aminoglycoside antibiotics, such as parenteral gentamicin, streptomycin, tobramycin and oral neomycin, during pregnancy. Pair analysis of cases with congenital abnormalities and matched healthy controls was carried out. The setting was the population-based dataset of the Hungarian Case-Control Surveillance of Congenital Abnormalities, 1980-96. In total, 38,151 pregnant women who had newborn infants without any defects (control group) and 22,865 pregnant women who had foetuses or newborns with congenital abnormalities were included in the study. 38 (0.16%) and 42 (0.11%) pregnant women in the case and control groups, respectively, were treated with the aminoglycosides studied. A teratogenic potential of gentamicin and neomycin was not indicated by a comparison of the occurrence of aminoglycoside antibiotic treatments in the total control group as referent with the figures of different congenital abnormality groups. In addition, the case-control pair analysis during the second-third months of pregnancy did not show a teratogenic risk of gentamicin and neomycin. The conclusion of this study is that treatment with parenteral gentamicin and oral neomycin during pregnancy presents no detectable teratogenic risk to the foetus, when restricted to structural developmental disturbances.