Quantitative indices are described for assessing the distribution of ventilation imaging agents (radioaerosols, in particular) within the lung. They have been applied to images obtained with ⁹⁹Tc^m aerosol particles (0.5-2.0 µm diameter) and with ⁸¹Kr^m gas in 12 patients with a wide range of lung function. In patients with normal lung function aerosol distribution was similar to that of ⁸¹Kr^m. In patients with airways obstruction, the aerosol tended to penetrate less well than ⁸¹Kr^m to the lung periphery and to show a less homogeneous distribution there. Quantitative analysis confirms that the aerosol technique is an effective substitute for ⁸¹Kr^m in patients with normal lung function. When lung function is impaired aerosol images may still be valuable particularly in confirming a diagnosis of pulmonary embolism though they must be interpreted with caution in attempting to exclude that diagnosis. It is concluded that small particle radioactive aerosols provide a readily available ventilation technique for clinical ventilation-perfusion imaging.