Overo Lethal White Syndrome (OLWS) is a condition that occurs in newborn foals. The condition is genetic, and both parents carry the defective gene. Horses that carry this gene are most commonly overo white patterned horses (frame overos), but there are exceptions. The defective gene has been found in

• American Paint Horses,
• American Miniature Horses,
• Half-Arabians,
• Thoroughbreds, and
• cropout Quarter Horses (foals born to registered Quarter Horse parents that have too much white to qualify for registration with the American Quarter Horse Association).

OLWS foals have blue eyes and are completely or almost completely white at birth. These foals initially appear normal except for their unusual coloring. After a varying period of time, troubling signs of colic emerge due to the foal's inability to pass feces. The OLWS foal has an underdeveloped. There is no treatment for OLWS, and surgery to bypass the intestinal damage has never been successful due to the extensive nature of this type of lesion. Veterinarians advise euthanasia for all OLWS foals because death will inevitably occur from colic caused by fatal constipation.