The reasons a cat's third eyelid is showing can be benign, but it can also be a symptom of a serious disease. These can be bacterial infections or other diseases which affect the eye directly, but they can also be more systemic. Also known as the nictitating membrane, the third eyelid can appear in one eye or both, depending on the nature of the problem. Other concurrent symptoms can help us to determine the cause of the third eyelid showing. It is also possible for a cat third eyelid showing with no other symptoms. If you are at all unsure, you will need to take the cat to a veterinarian for diagnosis.At AnimalWised, we look at the causes and treatment of cat third eyelid showing. We stress that it is important this problem is not ignored, especially if we observe other symptoms which indicate a local or systemic illness.