Clinically unrecognized intravascular volume overload may contribute to worsening symptoms and disease progression in patients with chronic heart failure (CHF). The present study was undertaken to prospectively compare measured blood volume status (determined by radiolabeled albumin technique) with clinical and hemodynamic characteristics and patient outcomes in 43 nonedematous ambulatory patients with CHF. Blood volume analysis demonstrated that 2 subjects (5%) were hypovolemic (mean deviation from normal values −20 ± 6%), 13 subjects (30%) were normovolemic (mean deviation from normal values −1 ± 1%), and 28 subjects (65%) were hypervolemic (mean deviation from normal values +30 ± 3%). Physical findings of congestion were infrequent and not associated with blood volume status. Increased blood volume was associated with increased pulmonary capillary wedge pressure (p = 0.01) and greatly increased risk of death or urgent cardiac transplantation during a median follow-up of 719 days (1-year event rate 39% vs 0%, p <0.01 by log-rank test). Systolic blood pressure was significantly lower in hypervolemic patients than in those with normovolemia or hypovolemia (107 ± 2 vs 119 ± 2 mm Hg, p = 0.008), and hypotension was independently associated with increased risk of hypervolemia in multivariate analysis (odds ratio 2.64 for a 10-mm Hg decrease in systolic blood pressure, 95% confidence interval 1.13 to 6.19, p = 0.025). These findings demonstrate that clinically unrecognized hypervolemia is frequently present in nonedematous patients with CHF and is associated with increased cardiac filling pressures and worse patient outcomes.