In order to assess the effect of steroid on thallium-201 uptake by glioma, 201Tl single-photon emission tomography was performed before and after steroid administration in four patients with recurrent malignant glioma. After steroid administration the 201Tl index, expressed as the ratio of 201Tl uptake in the tumour to that in the contralateral cerebral hemisphere, was 0.77+/-0.11 of the value before steroid (mean+/-SD: P<0.05 by paired t test). The 201Tl index has been used as a possible indicator for the differentiation of malignant gliomas from relatively benign tumours or radiation necrosis. The present results indicate that the effect of steroid has to be taken into account when semi-quantitative analysis, e.g. by means of the 201Tl index, is used in patients with brain tumours.