Three male patients (aged 41, 71 and 65 years) with untreated severe hypothyroidism of long duration were in a state of coma. This had been preceded by respiratory symptoms accompanied by hypoxemia and hypercapnea. There were also various chest radiographic findings. All the patients were treated with mechanical ventilatory assistance, circulatory support and i. v. administration of high doses of l-triiodo-thyronine (T3) during the early period of treatment. None of the patients showed any serious cardiac ill-effects immediately or a few days following the intravenous administration of T3. In spite of the stabilisation of vital signs and improvement in arterial oxygenation, the patients remained in a comatous or semicomatous state for the first few days. During the second week, the condition of their lungs deteriorated and prolonged respiratory failure ensued with fever, rales, excessive bronchial secretion, homogeneous densities in the chest radiograms and persistent hypoxemia even though mechanical ventilatory treatment was continued. The implications of this delayed respiratory failure during the treatment of myxedema coma are discussed.