Although the relationship between blood pressure and cardiac performance has been widely recognized, there are few published clinical observations concerning the effect of blood pressure control on cardiac function. We evaluated the effect of prazosin, an antihypertensive agent which also improves hemodynamic measurements in normotensive patients with heart failure, in 16 patients with moderate hypertension and reduced ejection fractions. Therapy with digoxin and diuretics was continued throughout the study, but other antihypertensive agents were withdrawn at least one week prior to the initiation of the study. Measurements of ejection fraction, cardiothoracic ratio and the duration of maximal treadmill exercise were made before and after two months of antihypertensive therapy with prazosin. On prazosin, blood pressure fell from a mean of 169/103 to 141/84. Excellent control was achieved in 13/16 patients and significant reductions were noted in the remaining three. Concomitantly, ejection fraction rose from .38 +/- .02 (SEM) to .43 +/- .03 (P less than .02), cardiothoracic ratio decreased from .55 +/- .02 to .53 +/- .02 (P less than .05) and exercise capacity increased from 9.2 +/- 0.9 to 11.9 +/- 1.1 minutes (P less than .005). Prazosin was well tolerated except in one patient who experienced worsening angina. These findings emphasize the importance of rigorous blood pressure control in hypertensive patients with left ventricular dysfunction and indicate that prazosin is effective in this setting.