A heart muscle disease in the Boxer dog was initially documented in the 1980s and referred to as Boxer cardiomyopathy. Studies have confirmed that this is an inherited disease primarily characterized by an electrical disturbance in the heart that may lead to collapsing episodes and sudden death. There is increasing demand for a screening test that could be used to evaluate dogs for the disease before they are selected for breeding purposes. Unfortunately, many of the clinical abnormalities do not become apparent until the dog is several years old. Therefore, there is significant interest in developing a DNA test that could be performed before a dog is selected for breeding. The study proposed here is a continuation of a study in which 268 Boxers were evaluated by physical examination, echocardiography and ambulatory electrocardiography. Dogs have been classified as affected, equivocally affected or unaffected; pedigrees and DNA samples have been collected. Three-generation families have been identified. Linkage and candidate gene analysis will now be performed using canine markers and these DNA samples. The identification of a genetic marker linked to familial ventricular arrhythmias will be the first stem pin the development of a DNA screening test.