We retrospectively studied 11 instances of patients requiring prolonged mechanical ventilation. Their spontaneous ventilatory measurements were not useful in judging their ability to wean, since these measurements did not change from the period of unsuccessful weaning to the period of progressive weaning from the ventilator. An adverse factor score and a ventilator score were created to evaluate underlying medical and respiratory problems related to ability to wean. Each score and the sum of the two scores separated patients between unsuccessful and successful weaning periods. We also found that the course and the duration of the entire weaning process could be predicted once progressive weaning had begun. We conclude that the adverse factor score and ventilator score correlate with the ability of patients receiving prolonged mechanical ventilation to wean.