The 1980-1984 data base of the Hungarian Case Control Surveillance System for Congenital Anomalies was used to evaluate possible teratogenicity of allylestrenol therapy during pregnancy. In an initial global analysis, three of the 24 congenital anomaly groups studied (ie, clubfoot, multiple anomalies, and hypospadias) had a significantly higher incidence of allylestrenol use. A case control analysis, however, excluded a pathogenetic role for allylestrenol in the etiology of clubfoot and multiple congenital anomalies. A greater use of allylestrenol in the first global evaluation was explained by a higher incidence in these groups of impending miscarriage and preterm labor, which are indications for allylestrenol therapy. The case control analysis did indicate a greater use of allylestrenol in the hypospadias group, but this difference was not statistically significant in the critical period for induction of hypospadias (ie, the third and fourth months of gestation). The causal role of subfertility in the etiology of hypospadias was an indirect factor, explaining the greater use of allylestrenol during pregnancy in this group; in Hungary, women with a history of infertility frequently receive hormonal support in the first trimester. The authors conclude that the data analyzed do not indicate any teratogenic effects of the use of allylestrenol during pregnancy.