In general the 123I-IMP SPECT image of a brain tumor is visualized as a defect. Tumor cases with high IMP uptake have been rarely reported. We encountered a case of primary malignant lymphoma of the brain with high IMP uptake on both early and late scans corresponding to the CT lesions. Previously reported positive cases seemed to be confined to the early scan. For the present the exact behaviour of IMP is incompletely understood, but the mechanism of high uptake has been accounted for by increased extraction and increased amine receptors at the tumor sites. We need further information to establish whether these SPECT findings are specific for primary malignant lymphoma of the brain.