The incidence of intrauterine fluid collections during dioestrus (12/43, 28%) and uterine cysts throughout the oestrous cycle (11/73, 15%) found in this study indicates that these ultrasonically detectable abnormalities are prevalent in mares. The hypothesis that uterine cysts do not affect pregnancy was not supported. Intrauterine fluid collections at dioestrus represented the presence of an inflammatory process as indicated by a high biopsy score, reduced progesterone concentrations, and a shorter interovulatory interval. Mares with fluid collections at dioestrus had a lower pregnancy rate at Day 11 and a higher embryonic loss rate by Day 20 than did mares without such collections. The progesterone profile and length of interovulatory interval for mares with uterine inflammation supported the hypotheses that embryonic loss in this herd was due to uterine-induced luteolysis rather than primary luteal inadequacy.