Hypothyroidism rarely complicates pregnancy because most affected women are anovulatory. In this report, we describe 28 complicated pregnancies cared for over a ten-year period at Parkland Memorial Hospital. In the group of 16 pregnancies in 14 overtly hypothyroid women, maternal complications were common and included anemia (31%), preeclampsia (44%), placental abruption (19%), postpartum hemorrhage (19%), and cardiac dysfunction. Perinatal morbidity and mortality were also high mainly because of placental abruption, and reflected frequent low birth weight (31%) and fetal death (12%). In a group of 12 women with subclinical hypothyroidism, these complications were less impressive. We speculate that overt thyroid deficiency is associated with adverse pregnancy outcome related to preeclampsia and placental abruption. Thyroxine replacement probably improves these outcomes even if subclinical hypothyroidism persists.