In tomographic Technegas ventilation studies, the deposited aerosol should be stable. The aim of this study was to investigate the stability of deposited Technegas in severely obstructed patients and in healthy volunteers. Six emphysematous patients and five healthy volunteers were studied. Anterioposterior (AP) and posterioanterior (PA) dynamic images of ventilation (120 frames, 1 frame.min-1) were acquired after inhalation of 99mTc-Technegas' in the supine position. The time-activity curve of the whole lung showed two phases in the emphysematous patients as well as in the healthy volunteers. The activity decreased rapidly in the first phase, then reached almost a constant level (the second phase) after approximately 50 min. The logarithms of both the first phase and the second phase were close to straight lines. The half-time of the first phase was approximately 9 min. No systematic differences in 'Technegas' clearance rate were found between central regions and peripheral regions and between high count rate regions and normal count rate regions. Pertechnegas contamination was subsequently detected, and we conclude that the rapid first phase is probably due to clearance of Pertechnegas. When stable Technegas is required, we suggest that data acquisition should start about 50 min after inhalation.