Hypoluteoidism is characterized by insufficient secretion of progesterone by the corpora lutea during pregnancy. The resulting failure to maintain progesterone concentration above a critical level presumably could lead to fetal resorbtion as well as frank abortion. This report concerns a 2.5-year-old Bernese Mountain dog with a history of two previous pregnancies ending in abortion around Day 50 of pregnancy. The bitch was initially presented 2 days after mating. Physical and gynecological examination revealed no abnormalities. The infectious causes of abortion in the bitch, Brucella canis and herpesvirus, were excluded using serology. On Day 26 after mating, ultrasonography confirmed a pregnancy with at least four living fetuses. During the remaining part of the pregnancy repeated transabdominal ultrasonography and plasma progesterone measurements, using a RIA, were performed. On Day 42, ultrasonography revealed living fetuses but plasma progesterone concentration had decreased to 8.3 nmol/L, which is just above the threshold necessary to maintain a vital pregnancy. Oral treatment with 0.1mg medroxyprogesterone acetate (MPA) per kg body weight, once daily, was started and continued until Day 58 in order to prevent abortion due to progesterone deficiency. The endogenous plasma progesterone concentration decreased further, but pregnancy was maintained. On Day 59 a cesarean section was performed because of dystocia and four living and one dead pup were delivered. One puppy had severe facial malformations and was euthanised. The premature decrease in plasma progesterone concentration while ultrasonography demonstrated that the fetuses were still alive, and the maintenance of pregnancy during administration of MPA, strongly support the diagnosis of hypoluteoidism.