Systolic heart murmurs caused by chronic mitral valve disease are particularly common in Cavalier King Charles Spaniels (CKCS) in Great Britain. To determine if American-bred CKCS have a similar high prevalence of chronic valve disease, results of stethoscopic examinations on 394 CKCS were analyzed. Left apical systolic heart murmurs were found in 22% of the dogs. The prevalence ranged from 9% in dogs < 1 year old to 100% in those > or = 10 years old; prevalence was 56% in dogs > or = 4 years old. Differences were not found in prevalence between sexes and among various coat colors. Reexamination of 79 dogs after 1 year revealed an incidence of new murmurs of 21%. Comparison of ages at initial examination in 128 referral hospital cases with chronic mitral valve disease revealed a mean age of 6.25 years in 17 CKCS, in contrast to a mean age of 12 years in other breeds. Echocardiographic and necropsy findings indicated that ruptured chordae tendineae and mitral valve prolapse are major components in the chronic valve disease process in CKCS.