If your cat really had nine lives, he probably wouldn't want to waste one of them on avocados, no matter how delicious they are. Fortunately, your cat with only one life is unlikely to die from a taste of avocado, but it may cause him some distress.
The problem with feeding avocados to cats, or letting them eat parts of an avocado tree, is that the plant and fruit contain a toxin called persin. It doesn't affect humans, and some varieties of avocados won't bother cats too much, but it's not a risk worth taking. The Guatemalan avocado variety contains high levels of persin and is known to cause problems for cats as well as other domestic animals such as dogs, horses and birds. The amount of persin can also vary depending on growing conditions and the time of year.
Luckily, avocados are only mildly toxic to cats unless consumed in large amounts. A healthy adult cat is not likely to die from eating avocado, but could have an upset stomach, including vomiting and diarrhea. Young kittens, seniors and cats with health problems could be more seriously affected. If your cat appears to have difficulty breathing or swallowing, has a high fever or rapid heartbeat, seek emergency treatment. Avocados can also be dangerous to cats if they eat the skin or chew on the pit since large chunks of these parts could cause an obstruction.
It's confusing to learn that avocados are toxic, and then see commercial cat products like food and shampoo that advertise avocado as a healthy ingredient. The difference is that these products use extracts from the avocado that do not contain persin. The avocado oil or extract used in cat food is safe for cats to ingest.
If your cat does happen to sink his teeth into an avocado, or chews on the leaves, stems or bark of an avocado tree, call your veterinarian right away or contact the Pet Poison Helpline at (800) 213-6680. Small amounts of avocado are unlikely to cause serious problems for your cat, but it's better safe than sorry. Your vet or the poison helpline staff will tell you what symptoms to watch for, whether any home treatments should be given and whether your cat needs emergency medical care.