Emission computed tomography has achieved considerable credibility in thallium-201 cardiac imaging and in brain imaging studies. To date the procedure has had only limited application in the study of other organs. In the present study traditional perfusion lung scans were compared with horizontal, sagittal and frontal tomograms in 30 patients. Preliminary results showed agreement between the two methods in most cases. Tomography, however, gave more precise information about the site and extent of areas of deficient lung perfusion. In a small number of cases tomography also revealed defect not seen on conventional scans. In patients with cardiomegaly in whom conventional scanning data were difficult to assess, tomography proved enlightening.