Abstract Objective—To characterize sleeping respiratory rates (SRRs) and resting respiratory rates (RRRs), collected in the home environment, of dogs with subclinical heart disease that could result in left-sided congestive heart failure. Design—Prospective cross-sectional study. Animals—190 adult dogs with subclinical left-sided heart disease. Procedures—Most dogs had mitral valve disease or dilated cardiomyopathy of various severities. Clients collected ten 1-minute SRRs or RRRs during a period ranging from 1 week to 6 months. Clinicians provided echocardiographic and medical data on each patient. Results—The within-dog mean SRR (SRR_mean; 16 breaths/min) was significantly lower than the within-dog mean RRR (RRR_mean; 21 breaths/min). Seven dogs had SRR_mean and 33 dogs had RRR_mean > 25 breaths/min; 1 dog had SRR_mean and 12 dogs had RRR_mean > 30 breaths/min; these dogs mostly had a left atrial (LA)-to-aortic ratio > 1.8. Dogs with moderate LA enlargement had a significantly higher SRR_mean than did other dogs. However, median SRR_mean for each of 4 levels of LA enlargement was < 20 breaths/min; median RRR_mean for each of 4 levels of LA enlargement was < 25 breaths/min. Both within-dog SRR and RRR remained stable for 10 consecutive measurements. Treatment with cardiac medications or presence of pulmonary hypertension was not associated with SRR_mean or RRR_mean. Conclusions and Clinical Relevance—Results suggested that dogs with confirmed subclinical left-sided heart disease of various severities generally had SRR_mean < 25 breaths/min, which was infrequently exceeded at any time, and that SRR and RRR remained stable, regardless of individual within-dog SRR_mean or RRR_mean. (J Am Vet Med Assoc 2013;243:839–843)