A 9 year old multiparous, standardbred broodmare presented to Cornell University Hospital for Animals in late gestation for progressive abdominal enlargement, ventral edema, inconsistent appetite and lameness. On transabdominal ultrasound, a single active fetus with a normal heart rate was found, and the allantoic fluid depth was markedly increased. On transrectal palpation the fetus was difficult to feel due to the increased allantoic fluid. The mare was diagnosed with hydroallantois, a rare condition in horses. The mare remained hospitalized for management and monitoring during the remainder of her pregnancy. At 338 days of gestation an assisted delivery was performed. A live foal was born with some signs of dysmaturity. The mare was treated for retained fetal membranes and inadequate milk production. This unusual case demonstrates many of the diagnostic and monitoring skills necessary for management of high-risk equine pregnancies.