OBJECTIVE. This study was performed to assess the accuracy of determining lung volume in patients with emphysema using MR imaging and then to investigate changes in thoracic dimensions after lung volume reduction surgery. SUBJECTS AND METHODS. Fast gradient-echo breath-hold MR imaging through the entire thorax at full inspiration and expiration was performed in 21 patients with severe emphysema and was performed again in nine of the patients who underwent surgery. Lung volumes were determined using a semiautomated computerized method of delineating the lungs and summing cross- sectional areas. These summed areas were compared with volumes measured on plethysmography and CT. Postoperative changes in thoracic structure were determined by measuring anteroposterior and transverse lung dimensions and lung height before and after surgery. RESULTS. The correlation coefficients and SEM for determining inspiratory lung volume were MR imaging versus plethysmography, r = .77, SEM = -12% (volume measured as less on MR imaging); CT versus plethysmography, r = .86, SEM = -13% (volume measured as less on CT); and MR imaging versus CT, r = .87, SEM = 4% (volume measured as greater on MR imaging). The correlation coefficients and SEM for determining expiratory volume on MR imaging versus plethysmography were r = .77, SEM = 6% (volume measured as greater on MR imaging). After surgery, decrease were found in all thoracic dimensions, and such decreases were greatest at expiration. CONCLUSION. MR measurements of lung volume are comparable with those of CT and differ from those of plethysmography. Changes in thoracic dimensions after lung volume reduction surgery are consistent with improved respiratory mechanics.