Background: Little information is available about the prevalence of renal dysfunction in dogs with chronic valvular heart disease (CVD). Hypothesis: Azotemia and a decrease in glomerular filtration rate (GFR) are more severe with increased severity of CVD. Animals: 124 (study No. 1) and 24 (study No. 2) client-owned dogs with CVD. Methods: A retrospective study (study No. 1) was performed to assess the prevalence of azotemia in the New York Heart Association (NYHA) classes of heart failure in dogs with CVD. A prospective study (study No. 2) was then designed to determine GFR in dogs with different degrees of CVD severity. Complete physical examination, electrocardiography, blood pressure measurement, thoracic radiographs, echocardiography, and plasma and urine analyses were also performed. Results: In study No. 1, 50% of the dogs were azotemic and the percentage of azotemic dogs increased with functional class (up to 70% in NYHA class IV patients). In study No. 2, 8/24 dogs were azotemic. Plasma urea and creatinine were higher in NYHA class III-IV dogs compared with class I-II dogs. The GFR was lower (P<.001) in NYHA class III-IV dogs (1.7+or-0.7 mL/min/kg) than in class I to II dogs (3.1+or-0.8 mL/min/kg). Only 1 dog in class I-II had a GFR below 2 mL/min/kg and only 2/9 class III-IV dogs had a GFR above 2 mL/min/kg. Conclusion and Clinical Relevance: Azotemia and renal impairment increase with the severity of congestive heart failure and are frequent findings in dogs with CVD. It remains to be shown if deterioration of renal function is a direct result of progression of the heart disease.