To define the costs and benefits associated with prolonged mechanical ventilation, we studied retrospectively the records of 137 consecutive patients who required at least 48 h of ventilator support. The patients were physiologically unstable and required intensive care. Causes of respiratory failure included pulmonary diseases, post-operative complications, neuromuscular diseases, cardiac dysfunction, and GI disease. Forty-nine (36%) patients survived the hospitalization, and 38 (28%) patients were alive 3 yr after receiving prolonged mechanical ventilation. The mean total hospital cost was +16,930/patient (U.S. dollars, 1976-1977). The cost-benefit averaged +1826/yr of extended life. These costs varied from +460/yr of extended life for patients with respiratory failure complicating asthma to +8026/yr for patients with cardiac dysfunction. The cost-benefit ratio increased sharply for men older than 56 yr and for women older than 75 yr. These data document the importance of the basic disease process and the patient's age in the cost-benefit relationship.