To evaluate the accuracy of a portable respiratory inductive plethysmograph that allows the monitoring of ventilation without airway instrumentation during exercise in unrestrained subjects. Validation of a novel technique by comparison to a reference standard. Thirty-one subjects, including 20 healthy volunteers, 6 patients with COPD, and 5 patients with congestive heart failure. Participants performed progressive treadmill exercise to exhaustion. Ventilation was monitored by a novel battery-powered, miniaturized, and calibrated respiratory inductive plethysmograph. Inductance sensors encircling the rib cage and abdomen were built into an elastic body garment. A pneumotachograph attached to a mouthpiece served as the reference method. Breath-by-breath comparisons between the inductance plethysmograph and pneumotachograph over the course of progressive exercise to exhaustion revealed no significant bias of respiratory cycle time, tidal volume (Vt), and minute ventilation. The corresponding limits of agreement (bias +/- 2 SDs) were +/- 6%, +/-17%, and +/- 17%, respectively, for 2,480 breaths. Comparisons of mean values averaged over 20 breaths revealed improved limits of agreement of +/- 1% for cycle time, and +/- 7% for tidal volume and minute ventilation, respectively, for 124 comparisons. Agreement between methods was similar for patients and healthy subjects. Among the patients, maximal minute ventilation was lower, and breathing was more rapid and shallow than in healthy subjects. Obstructive lung disease was associated with a shorter duty cycle than heart failure. The portable respiratory inductive plethysmograph accurately estimates ventilation during treadmill exercise, and identifies differences in breathing patterns among patients with pulmonary or cardiac diseases and healthy subjects. This unobtrusive monitoring technique is promising for application in ambulatory patients.