OBJECTIVE--To define patients with left atrial mechanical failure and identify its echocardiographic, physiological, and clinical associations. DESIGN--Prospective study with cross sectionally guided M mode and Doppler echocardiograms, and with apexcardiograms, electrocardiograms, and phonocardiograms. SETTING--Tertiary cardiac referral centre. PATIENTS--10 patients with left atrial mechanical failure and 20 healthy controls of similar age. RESULTS--10 patients with left atrial mechanical paralysis were identified among 4036 adults over a 1 year interval. Nine were in sinus rhythm and one had a DDD pacemaker. Left atrial mechanical activity was absent on M mode echocardiograms of the left sided atrioventricular ring and the aortic root. A Doppler A wave on transmitral flow and a clearly defined A wave on the left ventricular apexcardiogram were also absent, though evidence of right atrial mechanical movement was present in nine patients. Mean (SD) age was 63 (19) years and six were men. Nine had left ventricular disease and one had undergone extensive resection of the left atrium. Abnormal measurements of left ventricular end diastolic dimension (62 (13) mm), fractional shortening (15 (6)%), isovolumic relaxation time (19 (12) ms), left atrial size (45 (10) mm), and transmitral Doppler E wave deceleration time (110 (35) ms) were recorded. CONCLUSION--Left atrial mechanical failure may be present in patients with left ventricular disease despite normal sinus rhythm. Normal atrial activation on 12 lead electrocardiogram suggests it is primarily mechanical in origin. The possibility of left atrial mechanical failure must be considered when Doppler patterns of transmitral flow are used to assess left ventricular diastolic function.