This is my son Alex when he was younger sitting on our old Bullmastiff "Cody". I know. The Bullmastiff looks pretty impressive, but as a breed, they are not my "cup of tea". Cody was a very sickly dog. He died of an immune system deficiency problem which allowed many illnesses to effect him during the course of his four years on earth. Many Bullmastiffs are short lived.
Cody was useless as a guard dog too. He was a huge, friendly, slob of a sickly thing and that's what he was. If this is what you want, enjoy Bullmastiff ownership. They are a sweet breed, anyway. I demand more than that of a dog.
