The possible teratogenic effect of levamisole treatment during pregnancy. Levamisole (Decaris) is an anthelminthic agent. The teratogenic effect of levamisole was evaluated by the analysis of a large population-based data set of the Hungarian Case-Control Surveillance of Congenital Abnormalities, 1980-1996. The mean gestational age was somewhat longer after levamisole treatment (adjusted t = 1.04; p = 0.30) while the mean birth weight was nearly similar in the two study groups (adjusted t = 0.36; p = 0.72). This first controlled case-control epidemiological study does not indicate a higher rate of congenital abnormalities in children born to mothers who had received oral levamisole treatment during pregnancy.