OBJECTIVE: To evaluate the consequences of maternal thyroid dysfunction for pregnancy outcome. METHODS: A retrospective analysis involving all pregnant women who delivered in Slovenia in the 1997-1999 triennium; those having a medical history of thyroid dysfunction and/or taking thyroid medications were allotted to the study group (n = 748) and the remaining ones to the control group (n = 52 253). RESULTS: Significantly higher incidences of infertility (5.5% vs. 3.7%, p < 0.05), menstrual cycle irregularities (3.2% vs. 1.9%, p < 0.05), hypertensive disorders (7.0% vs. 4.2%, p < 0.05), threatened preterm delivery (9.1% vs. 5.6%, p < 0.001), and delivery before 32 weeks (2.7% vs. 1.5%, p < 0.05) were found in the study than in the control group. There were no significant differences in the incidences of miscarriage, non-gestational diabetes mellitus, proteinuria, hyperemesis, intrahepatic cholestasis of pregnancy, intrauterine growth restriction, placental abruption, preterm delivery, small for gestational age newborns (SGA), and stillbirths. CONCLUSIONS: This is the first study to evaluate the incidence of thyroid dysfunction for the whole population of pregnant women in Slovenia using a retrospective analysis. Thyroid dysfunction adversely affects pregnancy and pregnancy outcome but to a lesser extent than presented in previous studies. An evaluation of thyroid function in the women who experience menstrual cycle irregularities, infertility, and complications during pregnancy, labor and delivery would be advisable.