Bleeding from the nostril or nostrils can be a sign of many conditions, originating anywhere from the external nostril to deep in the lungs. The most common cause of nasal bleeding is ethmoid hematoma, a slow-growing mass in the nasal passage or sinus. Less common but more severe problems include hemorrhage from the guttural pouch or a fracture into the guttural pouch or sinus.

In very dry conditions, minor bleeding from the nasal passages can occur. Usually, blood comes from the nasal passages and not the external nostril. Horses can also traumatize the thin mucous membranes of the inside of the nostril, but this is relatively rare.

Racehorses that are bleeding after a race are bleeding from the lungs (Exercise Induced Pulmonary Hemorrhage), unless proven otherwise.

Horses with clotting disorders can bleed from the nostrils, but usually show other symptoms of disease.