OBJECTIVESTo describe left atrial size, left atrial volume, left atrial function and left ventricular diastolic function in healthy cats and those with hypertrophic cardiomyopathy without and with congestive heart failure.METHODSA retrospective study of 61 client‐owned, 21 healthy, 21 asymptomatic hypertrophic cardiomyopathy and 19 with hypertrophic cardiomyopathy and congestive heart failure cats. Data were retrieved from clinical records and echocardiography archives. Left atrial diameter and volumes were measured. Left atrial function was investigated using changes in diameter (fractional shortening) and volume (Simpson's method; left atrial ejection fraction). Conventional echocardiographic indices of left ventricular diastolic function were recorded.RESULTSLeft atrial diameter and left atrial volume measurements were significantly higher in hypertrophic cardiomyopathy with congestive heart failure cats compared with asymptomatic hypertrophic cardiomyopathy and healthy cats (P < 0 · 001). Left atrial passive, active and complete ejection fraction distinguished between hypertrophic cardiomyopathy with congestive heart failure and asymptomatic hypertrophic cardiomyopathy (P < 0 · 001). Hypertrophic cardiomyopathy with congestive heart failure cats had significantly lower mitral A wave velocity (P = 0 · 016) and atrial complete emptying based on diameter and volume measurements (P = 0 · 008 and P < 0 · 001, respectively) compared with asymptomatic hypertrophic cardiomyopathy cats.CLINICAL SIGNIFICANCELeft atrial volume is obtainable by echocardiography in cats. Left atrial volume and atrial function may indicate chronicity and severity of diastolic dysfunction associated with hypertrophic cardiomyopathy and congestive heart failure. Left atrial function was reduced in cats with hypertrophic cardiomyopathy and congestive heart failure compared with healthy and asymptomatic hypertrophic cardiomyopathy groups.