The effects of malnutrition and refeeding on nutritional indices, pulmonary function, and diaphragmatic contractile properties were studied in severely malnourished patients with anorexia nervosa. Fifteen patients were evaluated upon hospital admission (Day 0) and on Days 7, 30, and 45 after starting feeding. Spirometry, lung volumes, and arterial blood gases were measured at each time interval, as were contractile properties of the diaphragm as assessed by transdiaphragmatic pressure generated during electrical phrenic nerve stimulation (Pdistim) and a maximal sniff maneuver (Pdisniff). Anthropomorphic and biochemical measurements were performed at each time interval. Patients were severely malnourished upon admission; mean body weight was 37.1 +/- 4.7 kg (63% ideal body weight). During nutritional support, body weight increased significantly to 42.9 +/- 4.6 kg on Day 45 (p < 0.01), as did muscle mass: 11.2 +/- 4.1 kg on Day 0, to 16.6 +/- 4.9 kg on Day 45 (p < 0.01). Vital capacity and FEV1 increased significantly by Day 30 (p < 0.05). Lung volumes were unchanged. Mean arterial blood gas values were also within the normal range at Day 0; PaO2, 92.6 +/- 2.4 mm Hg and PacO2, 41.0 +/- 1.5 mm Hg. Four patients, however, had an increased PacO2 (> 42 mm Hg) at Day 0, which returned to normal by Day 30. Diaphragmatic contractility was severely depressed initially; Pdistim, 15.9 +/- 1.4 cm H2O; Pdisniff, 65.4 +/- 5 cm H2O; but it significantly increased with nutritional support by Day 30 to 22.5 +/- 1.9 and 84.6 +/- 4.7 cm H2O, respectively. We conclude that diaphragmatic function is severely impaired in malnuorished patients free of other coexisting