Hyperaldosteronism has been causally linked to myocardial interstitial fibrosis experimentally, but it remains unclear if this link also applies to humans. Thus, we investigated the effects of excess aldosterone due to primary aldosteronism (PA) on collagen deposition in the heart. We used echocardiography to estimate left ventricular (LV) wall thickness and dimensions and for videodensitometric analysis of myocardial texture in 17 consecutive patients with PA and 10 patients with primary (essential) hypertension who were matched for demographics, casual blood pressure, and known duration of hypertension. The groups differed in serum K+, ECG PQ interval duration, plasma renin activity, and aldosterone levels (allP≤0.002) but not for casual blood pressure values, demographics, and duration of hypertension. Compared with hypertensive patients, PA patients showed a higher LV mass index (53.7±1.8 versus 45.5±2.0 g/m2.7;P=0.008) and lower values of the cyclic variation index of the myocardial mean gray level of septum (CVIs; −12.02±5.84% versus 6.06±3.08%;P=0.012) and posterior wall (−11.13±6.42% versus 8.63±9.62%;P=0.012). A regression analysis showed that CVIswas predicted by the PQ duration, supine plasma renin activity, plasma aldosterone, and age, which collectively accounted for ≈36% of CVIsvariance. PA is associated with alterations of myocardial textures that suggest increased collagen deposition and that can explain both the dependence of LV diastolic filling from presystole and the prolongation of the PQ interval.