You may have seen various articles on the web that report that avocado leaves may be toxic. I believe this topic is best explained by Diana Kennedy if her new book “From My Mexican Kitchen” just published, 2003:
"Because there has been some concern about toxicity of avocado leaves among some Californian aficionados, I think it is time to set the record straight. The toxicity reports relate back to a study done in 1984 at the University of California at Davis, which showed that dairy goats suffered some toxic effects from ingesting very large amounts of avocado leaves (the toxic agent remains unknown). The crucial point, according to Dr. Arthur L. Craigmill, toxicology specialist at Davis and one of the authors of the study, is that the toxic effects were traced to the Guatemalan avocado (Persea American). When the goats were fed Mexican avocado leaves (Persea dryminfolia), a different variety, there was no problem.
The Hass avocado, the best tasting one grown in America, is a hybrid of indeterminate origin though its DNA tests positive for a Guatemalan ancestor—hence the suspicions. No one has ever tested Hass leaves for toxicity, but it seems unlikely that the small amounts used in cooking would cause any problems in any case. When in doubt, choose based on tasted and that leads you to the aromatic Mexican leaves which are now available in the U.S."