After an initial safety study in 7 normal volunteers, a randomized double-blind comparison was made between 3,5-diiodothyropropionic acid (DITPA) and placebo in 19 patients with moderately severe congestive failure. In heart failure patients receiving the drug for 4 weeks, cardiac index was increased (p = 0.04) and systemic vascular resistance index was decreased (p = 0.02). Systolic cardiac function was unchanged but isovolumetric relaxation time was decreased significantly, suggesting improvement in diastolic function. Total serum cholesterol (p = 0.005) and triglycerides (p = 0.01) also were decreased significantly. DITPA could represent a useful new agent for treatment of congestive heart failure.