113mIn has been proposed as a suitable tracer for aerosol ventilation scintigraphy in combination with 99mTc pulmonary perfusion scintigraphy. The high energy (393 keV) of 113mIn allows it to be detected in the presence of 99mTc, but degrades the spatial resolution which can be achieved. We have compared the resolution obtained with 99mTc and 113mIn in phantom experiments and in aerosol ventilation scans in 14 patients with airways disease. The resolution at the distances from the collimator encountered in lung scintigraphy was 10-20 mm for 99mTc and 15-40 mm for 113mIn. Aerosol ventilation images were abnormal in all patients. The 99mTc images showed peripheral defects and frequently central hot spots reflecting increased local aerosol deposition. In the 113mIn images, the lower resolution had a smoothing effect, the central hot spots were less striking, and the distribution of activity appeared more uniform, some detail was lost in the periphery. It is suggested that 113mIn is useful for ventilation scintigraphy in the diagnosis of pulmonary embolism, but that 99mTc is the tracer of choice if aerosol ventilation scintigraphy is used to study airways disease.