Abstract: The data of the population-based Hungarian Case-Control Surveillance of Congenital Abnormalities between 1980 and 1996 were evaluated. The objective of the study was to check the effect of oral moroxydine, a biguanide antiviral drug during pregnancy for birth outcomes. First, the prevalence of moroxydine use in the mothers of cases with and controls without congenital abnormalities was compared. Of 38,151 pregnant women who delivered newborn infants without any defects (control group), 14 (0.04%), while of 22,843 pregnant women who had fetuses or newborns with congenital abnormalities, 13 (0.06%) were treated with the oral tablet of moroxydine (POR with 95% CI: 1.6, 0.7-3.3). The teratogenic potential of oral moroxydine treatment cannot be excluded because a higher use of moroxydine during the second-third month of pregnancy was found in the mothers of multi malformed cases and of anencephalic fetuses, though these possible associations were based only on two-two cases. Second, control newborn infants without any defect born to mothers with or without moroxydine treatment were compared. A somewhat larger mean birth weight (3.532±426 vs. 3.276±511, adjusted t = 1.8; p = 0.07) and a reduction of low birth weight (0 vs. 9.2%) was found in control newborn infants of the mothers with moroxydine treatment. The weak teratogenic potential and some birth weight promotion effect of moroxydine treatment during pregnancy may deserve some attention.