This study determines the characteristics of left atrial filling from the pulmonary veins in 80 patients with hypertrophic cardiomyopathy by transthoracic echocardiography. Patients in sinus rhythm (n = 72) showed a typical triphasic pattern with a systolic phase (J) and diastolic phase (K) and a retrograde surge of flow during atrial contraction. In sinus rhythm, three characteristic patterns of pulmonary venous flow in hypertrophic cardiomyopathy were observed: (1) an increased peak J wave relative to the K wave with J/K ratios greater than 1 (n = 43; 60%), (2) equal peak J and K phases (n = 7; 10%), and (3) decreased J wave relative to K ratios and J/K less than 1 (n = 22; 30%). The J/K ratio correlated inversely with the E/A ratio (r = -0.64; p < 0.01). In eight patients with atrial fibrillation, left atrial filling occurred primarily in diastole (K phase). Combined use of mitral and pulmonary venous flow may be optimal for the noninvasive detection of abnormal left ventricular filling in patients with hypertrophic cardiomyopathy.