Four at-risk Highland ponies on the same premises all retained their fetal membranes in the same breeding season. The ponies were treated with a combination of oxytocin, infusion and distension of the allantochorionic sac with fluid, gentle traction in one case, and supportive therapy. The possible causes of the condition were investigated with clinicopathological tests and analyses of the herbage for its species composition and fungal contamination. No obvious possible causative abnormalities were identified apart from a low total serum calcium level in one case. No fescue grass was found and no pathogenic fungi were detected.