Thallium-201 (Tl-201) imaging has been widely used to differentiate post-therapy reactions from residual viable tumour or local recurrence, However, the ability of Tl-201 to discriminate between tumour and posttherapy changes with superimposed infection/inflammation is unclear. This experimental study investigated the localization of Tl-201 in infected/inflamed tissues. Twenty-four rats infected with Staphylococcus aureus and 10 rats injected with a standard volume of saline solution (SS) into the thigh muscles were studied. Twenty-four hours after microorganism or SS administration, 18 MBq Tl-201 was injected intravenously. Images were recorded at 20 min and 3 h post-injection. The increased tracer uptake was evaluated qualitatively and quantitatively by calculating the ratios (L/C) derived from regions of interest drawn over the lesion and the contralateral thigh muscle. After the imaging procedure, histopathological examination was also performed. Whereas the control group showed no abnormal accumulation of activity, the infected rats demonstrated markedly increased activity, especially on the 20 min images. The mean L/C ratios for the 20 min and 3 h images for the infected rats were 2.18 +/- 0.20 and 1.52 +/- 0.04, respectively (P < 0.0005). In conclusion, positive uptake due to an infective process may limit the use of Tl-201 in studies monitoring response to tumour therapy. Although delayed imaging may help to overcome this limitation, further investigations among a large series of patients are required in order to improve the reliability of Tl-201 imaging in oncology.