Inter-observer variation in the detection and grading of low intensity heart murmurs in boxer dogs was investigated. Six veterinarians with different levels of experience examined 27 boxers by cardiac auscultation. The dogs were auscultated before and after exercise, and the results were compared with phonocardiographic and echocardiographic examinations performed at rest and during two different stress tests. A subvalvular aortic ridge was identified in six dogs on two-dimensional echocardiography. Using dogs with low intensity murmurs or dogs free of heart murmurs, inter-observer agreement was positively correlated to the level of experience at rest (weighted kappa [k] 0·14 to 0·75), while the agreement was poor after exercise (weighted k 0·01 to 0·36). The presence of a subvalvular aortic ridge was associated with higher aortic flow velocities (P<0·002) and higher auscultatory murmur grading (P<0·001). There was an increase in murmur duration during one kind of stress test (P<0·001) and in aortic flow velocity during the other (P=0·001).