The "traditional" weaning criteria, arterial blood gases, and a number of other physiologic variables were measured in 47 patients to evaluate to what extent reduced ventilatory reserves or extrapulmonary organ dysfunction affect successful extubation. All patients had been weaned from continuous positive airway pressure (CPAP) and from mechanical ventilation according to the intermittent mandatory ventilation (IMV) method; at the time of study, all patients had compromised arterial oxygenation relieved by supplemental inspired oxygen. No significant difference between patients successfully extubated and those who required reintubation was found using the traditional weaning criteria or blood gases. On the other hand, patients who required reintubation had significantly lower urine volume (p less than 0.01), lower respiratory quotient values (p less than 0.05), and a higher incidence of positive blood culture (p less than 0.05). These 3 variables also correlated best to the patients' outcome after extubation.