In humans and small animals, heart disease can lead to an increase in aldosterone, and the aldosterone level correlates with the severity of the heart disease. In horses similar interactions may be possible and may lead to an increase in aldosterone in horses with heart valve insufficiencies. In a prospective clinical trial eight healthy horses (control group) and 40 horses with heart valve disease were examined. In all horses, a clinical (auscultation), electro- and echocardiographic examination was performed and aldosterone plasma concentration was determined. The median aldosterone plasma concentration in the control group was 23.95 pg/ml. Twenty-one out of 40 horses with heart valve insufficiencies and without dimensional changes by echocardiography (group 1) showed a median aldosterone plasma concentration of 45.5 pg/ml. Five out of the 40 horses had a left atrial (LA) dilation and an average LA size with 147.6+/-11 mm (group 2) and a median aldosterone plasma concentration of 95.9 pg/ml. Five other horses had a left ventricular (LV) dilation with an average LV size of 141.6+/-6.8 mm (group 3) and a median aldosterone plasma concentration of about 115.3 pg/ml. In this group a positive correlation between aldosterone plasma concentration and LV existed (r=0.9, P=0.03). Nine horses with both LA (152.8+/-11.4 mm) and LV dilatation (145+/-9 mm, group 4) had a median aldosterone plasma concentration of 161.2 pg/ml. Significant differences of the aldosterone concentrations were observed between the control group and the horses with LA and LV dilation (group 4, P=0.0005), as well as between group 1 (horses with heart valve insufficiency but without dilation) and group 4 (P=0.0006). The study confirms that, as reported for other species, aldosterone rises as the severity of valvular disease increases. However, in this study, as there is only significant difference from normal in the most affected group, it will require further study before the plasma aldosterone level can be relied on as an indicator of the severity of heart disease in an individual horse.