In congestive heart failure captopril modifies the left ventricular filling pattern mainly by unloading the heart. We investigated whether the structural characteristics of the left ventricle may influence the acute effects of captopril on this pattern in patients with untreated hypertensive (H group, 6 patients) or idiopathic (I group, 14 patients) cardiomyopathy. We evaluated changes of pulsed Doppler mitral flow, of systemic arterial and wedge pulmonary pressures 40 min after 25 mg captopril administered sublingually, and correlated these changes with the M-mode echocardiographic relative wall thickness index (h/r). Baseline mean arterial pressure (H = 137 ± 20 mm Hg, mean ± SD, I = 95 ± 19 mm Hg; p < 0.001), and h/r (H = 0.38 ± 0.03,1 = 0.28 ± 0.09; p < 0.05) were greater in the high blood pressure group; wedge pressure, echocardiographic biplane ejection fraction, and Doppler indexes of the left ventricular filling were similar in the two populations. After captopril, ejection fraction did not change significantly, mean arterial pressure decreased significantly in hypertensive patients (H group, baseline = 137 ± 20, captopril = 119 ± 10, p = 0.02; I group, baseline = 95 ± 19, captopril = 90 ± 24, p = nonsignificant), and the wedge pressure was reduced by the same extent in both groups (H group, baseline = 27.7 ± 3, captopril = 21 ± 7, p < 0.05; I group, baseline = 20 ± 12, captopril = 15 ± 8, p < 0.05). In the H group early mitral flow increased [(E wave integral) × (mitral annulus area)] by 38 ± 15%, and was almost steady in the I group (-1.3 ± 30%; group H vs. I = p < 0.01); late mitral flow [(A wave integral) × (mitral annulus area)] showed a pattern exactly opposite to this (H = +0.4 ± 40%, I = +38 ± 19; p < 0.01). In the whole population there was a significant correlation between the early /late flow ratio variations and baseline h/r (r = 0.6, p < 0.05). In chronic congestive heart failure, changes in left ventricular filling with captopril are related to h/r: a higher index, as recorded in the H group, is associated with ‘true normalization’ of the filling pattern after captopril; a lower index, as recorded in the I group, is associated with ‘pseudonormalization’ despite a similar decrease of left ventricular filling pressure.