A single-photon emission tomography (SPET) technique for the absolute measurement of tumour perfusion is described. Phantom studies have shown that source-background ratios are dependent upon source size and radial position within the phantom. A means of correcting source-background count ratios for these variables has been developed and used to correct tumour-lung ratios obtained in 28 patients with bronchial carcinomas who underwent technetium-99m hexamethylpropyleneamine oxime (99mTc-HMPAO) SPET. On SPET images, the normal lung appears as a relatively homogeneous background. The relationship between 99mTc background concentration (kBq/ml) and counts/pixel was determined from phantom studies and the tumour 99mTc concentration from the background 99mTc concentration and corrected tumour-lung ratio. The total activity of the lipophilic 99mTc-HMPAO species injected was measured. The activity reaching the systemic circulation (Asys) was obtained by subtracting the activity trapped in the pulmonary circulation (obtained from background 99mTc concentration and lung volume). Tumour blood flow may then be calculated from fraction of Asys contained in the tumour provided cardiac output and extraction fraction are known. Blood flow through the central region of tumours ranged from zero to 59.0 (mean 14.1) ml min-1 100 g-1 and through the whole tumour from 0.6 to 68.0 (mean 20.6) ml min-1 100 g-1.