The study included 30 patients with hypertrophic cardiomyopathy (HCM) (obstructive and nonobstructive) and 15 control subjects. End-diastolic mitral annulus area (MAA(3D)) and mitral valve area (MVA(3D)) were measured by real-time 3-dimensional (3D) echocardiography. MVA(2D) and peak mitral inflow A wave velocity (V) were measured by 2-dimensional (2D) echocardiography. Left atrial ejection force (LA-EF) was calculated by 2D echocardiography and real-time 3D echocardiography using the formula: 0.5 x 1.06 x (MAA or MVA) x V2, where (1.06) is blood viscosity. LA-EF(2D-MVA), LA-EF(3D-MVA), LA-EF(3D-MAA), and V were significantly higher in patients with HCM than control subjects (P < .001). LA-EF(2D-MVA) and LA-EF(3D-MVA) were lower than LA-EF(3D-MAA) in HCM only (P < .001). In obstructive HCM, LA-EF(2D-MVA), LA-EF(3D-MVA), LA-EF(3D-MAA), and V were significantly higher than in nonobstructive HCM (P < .05). Left ventricular outflow tract gradient contributed independently to high LA-EF in obstructive HCM. We concluded that HCM is associated with higher LA-EF than normal, and higher in obstructive HCM than nonobstructive indicating a higher atrial workload that is reflected by LA-EF(3D-MAA).