To investigate the significance of augmentation of left ventricular slow filling, trans-mitral flow (TMF) was echocardiographically observed through pulsed Doppler method in 116 patients with hypertrophic cardiomyopathy (HCM), 74 with dilated cardiomyopathy (DCM), 27 with mitral regurgitation (MR), 86 with old myocardial infarction (OMI) and 80 normal controls (C). The slow filling-wave (SFW) in TMF was defined as the filling wave with obvious peak velocity during slow filling phase. The peak velocities (E, A, S) of the early filling, the atrial contraction and the slow filling waves were measured. The SFW was divided into two patterns according to the S and E ratio: large SFW (S/E greater than or equal to 1/2) and small SFW (S/E less than 1/2). 1) The small SFW was more frequently, but not significantly, observed patients with MR (37%) than in normal subjects (18%), patients with HCM (10%), DCM (4%) and OMI (7%). However large SFW was observed in 16 patients (14%) with HCM, but not in normal subjects and patients with other cardiac diseases excluding one patient with DCM. 2) In normal subjects and patients with DCM, OMI and MR, those with small SFW had larger E and smaller A/E than those without small SFW. However in patients with HCM, there was no difference in these indices (E and A/E) according to whether the patients were with or without SFW. 3) In HCM, patients with large SFW had significantly smaller E and significantly larger isovolumic relaxation time than those without SFW. Thus, appearance of the large slow filling wave, which might be caused by abnormal relaxation of the left ventricle, was frequently observed in patients with HCM.