Pulmonary embolism demands rapid and accurate diagnosis. And ventilation imaging has greatly improved the diagnostic accuracy of pulmonary embolism in addition to perfusion imaging. Agents currently used include xenon-133, krypton-8lm and technetium-99m radioaerosols. However radioactive gases are compromised by availability and cost for krypton-8lm, radiation dose, gamma energy and non-physiologic behaviour for xenon-133. Radioaerosols of technetium- 99m componds are rapidly cleared from the lung after inhalation, and their relative low effeciency (specific radioactivity) and wide distribution of particle sizes make them also suboptimum. A new ventilation agent, Technegas is a suspension of structured graphite ellipsoids with diameter below 20nm, labelled with Tc- 99m in a carrier gas of Argon. This report describes the authors' clinical experience with Technegas. This is the first reported clinical study of this agent in Korea. A comparison of Technegas and Tc-99m-DTPA aerosol was performed in 12 patients with various pulmonary diseases such as COPD, pulmonary tuberculosis and pleural effusion. All patients were studied with Tc-99m-DTPA aerosol inhalation and Technegas ventilation. In both studies image quality was assessed (1) semiquantitatively by scoring bronchial and gastric activity, (2) subjectivly by direct visual comparisan of peripheral lung images and (3) quantitatively by computing the peripheral penetration index(PI) for each lungs. The bronchial activites were seen in 7 out of 12 cases with Tc-99m-DTPA aerosol and in 5/12 with Technegas. The gastric activities were seen in 5/12 and 1/12 cases respectively. The average values of PI were 61.26% with Tc- 99m-DTPA aerosol and 69.20% with Technegas (p〉0.05). Using Tc-99m-DTPA aerosol, COPD patients showed deposition in the central airways with poor visualization of the peripheral areas of the lungs. In Technegas studies these phenomena were less prominent, and the examination is well tolerated by patient and requires only a minimum of patient cooperation. With superiority of easy availability and handling, better physical characteristics and favorable image quality, Technegas is a promising agent for lung ventilation scanning.