Objective: The purpose of this study was to define the cardiovascular abnormalities present in young and adult cats affected with the lysosomal storage diseases mucopolysaccharidosis (MPS) I and MPS VI. Method: Eighteen cats affected with MPS I and 10 cats affected with MPS VI were evaluated by physical examination, electrocardiography and echocardiography. Electrocardiography (ECG) was performed on all MPS I and 9 of the MPS VI cats. Twelve unaffected cats underwent complete examinations for comparison purposes. Results: No cardiovascular abnormalities were noted on physical examination. Measured ECG intervals were normal in affected cats; however, sinus arrhythmia was noted more frequently than in the unaffected cats. Significant echocardiographic abnormalities included aortic valve thickening, regurgitation and aortic root dilation. Significant mitral valve thickening was also noted. The severity of changes increased in older affected cats. Conclusion: As affected animals increased in age, more cardiac abnormalities were found with increasing severity. Significant lesions included the mitral and aortic valves and ascending aorta, but myocardial changes were not recognized. MPS I and MPS VI cats have similar cardiovascular findings to those seen in children and constitute important models for testing new MPS therapies.