Pulmonary function was investigated in eight patients with hypopituitarism in order to determine if the lung is affected by the generalized visceral atrophy of hypopituitarism; six patients with acromegaly and trophic hormone deficiencies were studied for comparison. The patients with hypopituitarism, including one with isolated growth hormone (GH) deficiency, had a restrictive type of ventilatory impairment (total lung capacity was 76.7 +/- 4.7 per cent of predicted, p less than 0.005) which was not influenced by cortisone, thyroxine or sex hormone replacement therapy. In contrast, the patients with acromegaly, despite deficiency of one or more pituitary hormones, had larger lungs than controls (total lung capacity was 119.5 +/- 5.0 per cent of predicted, p less than 0.01). These findings indicate that GH influences the lung volume in adult man and that the loss of GH secretion is likely to be responsible for the restrictive ventilatory impairment associated with hypopituitarism. Further studies showed that this restrictive defect was not related to neuromuscular impairment or to an abnormality of chest wall mechanics, and suggested that the ventilated air spaces retained normal elastic properties. It appears that a decrease in lung size occurs in patients who develop GH deficiency and thus, that normal levels of GH are necessary for maintaining normal lung size during adult life.