Studies of thoracoabdominal motion using the respiratory magnetometer were performed in 30 patients with chronic obstructive pulmonary disease. Volume equivalency of thoracic and abdominal deflections was established by using the concepts and methods developed by Konno and Mead. Twenty patients were ambulatory, although disabled, and 10 were in acute respiratory failure and were studied in a respiratory intensive care unit. Five of 20 ambulatory patients and 8 of 10 patients in acute respiratory failure showed inward abdominal motion coincident with outward rib cage motion during inspiration, suggesting ineffective diaphragmatic function. This pattern of thoracoabdominal motion was identical to that seen in 2 high quadriplegics with diaphragmatic paralysis when they were breathing entirely with their neck muscles. Inspiratory ascent of the diaphragm was confirmed fluoroscopically in 3 of the 5 ambulatory patients. Patients showing this pattern were generally severely disabled and had the largest residual volumes. Two abnormal patterns of thoracoabdominal motion were observed during the performance of maximal voluntary ventilation in the ambulatory patients. The first, seen in 9 of 20 patients, was characterized by reciprocal or paradoxical motion of rib cage and abdomen, with increase in rib cage volume associated with decrease in abdominal volume during inspiration. The second pattern, seen in 5 of 20 patients, showed complete disorganization of rib cage and abdominal motion, with no consistent or reproducible pattern. Thus, a significant proportion of patients with disabling chronic obstructive pulmonary disease show abnormalities in thoracoabdominal motion that are observable with the respiratory magnetometer and ofter by simple inspection. Most of these abnormalities suggest malfunction of respiratory muscles, particularly the diaphragm.