Addison's Disease:

Addison's disease is a very rare hormonal disorder of the adrenal glands.
It has been diagnosed in both European and American Leonbergers. It is serious and can lead to death if undiagnosed.
However, if diagnosed correctly, it can be very successfully managed with medication. Affected dogs often have periodic vomiting and diarrhea, lethargy, exercise intolerance, and weight loss.
Symptoms come on gradually and become worse with time, but in general it becomes drastic in 3 to 4 weeks time. It can be definitively diagnosed with blood tests.

The Leonberger Club of America is involved with on-going research into finding the cause of Addison's Disease.