Reasons for performing the study Embryo transfer (ET) is an increasingly common component of commercial sport‐horse breeding. While many variables affect the success of ET, optimal selection of recipient mares is essential. Ideally, adequate corpus luteum (CL) function would be evaluated by measuring plasma progesterone (P4) concentrations (≥2.0 ng/ml). In practice however, P4 is rarely measured because it would increase costs, and there is a delay to obtaining a result. Colour‐flow Doppler sonography (CF) has been described as a rapid means of assessing CL function because luteal blood flow correlates with circulating P4 concentrations. Objectives To use CF to assess luteal blood flow at the time of ET in recipient mares, and assess its relationship to P4 concentrations and pregnancy outcome. Study design Cohort study. Methods Fifty‐eight Day 8 horse embryos were transferred as part of a commercial programme. B‐mode and CF sonographic examinations were performed immediately prior to ET (ET+0), and blood was collected to measure plasma P4. Measurements were repeated at 4, 11, 18 and 25 days after ET in pregnant mares. Mares scanned not pregnant at Day 4 were re‐checked 2 days later. The CF image of each CL was frozen at peak total area of flow, and repeated 3 times. The cross‐sectional area (corrected for presence of lacunae) and the total area of colour pixels within the CL were analysed using ImageJ software. Results Plasma P4 correlated positively (P<0.05) with area of colour pixels (range r = 0.40–0.55) and total CL area (range r = 0.41–0.51) at all time‐points. None of plasma P4, CL area, or CF pixel area were predictive of pregnancy outcome. Conclusions CL blood flow is not predictive of pregnancy outcome but should help identify mares with an inadequate CL and not therefore suitable for receiving an embryo. Ethical animal research: Data collected during procedures performed as part of routine management. Explicit owner informed consent not stated. Sources of funding: Institutional resident research funding. Competing interests: None.