The usefulness of 99mTc-labelled serum albumin aerosol as a ventilation agent was compared to that of 81mKr gas in normal subjects and in patients with a wide range of pathological pulmonary function. In addition to visual analysis of the scintigrams, lung activity distribution was quantified by an index. In healthy subjects aerosol distribution was very similar to that of 81mKr gas. In patients with persistent obstruction of bronchial air flow, the aerosol tended to penetrate less well than 81mKr gas to the lung periphery and to show a more uneven pulmonary tracer distribution. Consequently the 81mKr gas technique resulted in underestimating bronchial obstructive defects compared with the radioaerosol method. The results indicate that unlike 81mKr gas imaging, small-particle aerosol scintigraphy provides a useful diagnostic technique for the detection of even mild air flow obstructions.