Radioaerosol inhalation lung scintigraphy is discussed and its interpretation based on the segmental anatomy of the lung is presented. Correlation between bronchography and inhalation scintigraphy in 26 cases of bronchogenic carcinoma showed that partial obstruction of a major bronchus produces a clear "hot spot" with peripheral decrease of aerosol deposition, while partial or complete obstruction of the segmental bronchi causes segmentally decreased deposition distal to the site of obstruction. Even at an early stage, bronchogenic carcinoma may be detected by aerosol inhalation scintigraphy whenever the tumors are large enough to produce visible abnormalities of aerosol deposition.