Twenty-five patients with malignant astrocytoma, either postoperatively (15 cases) or with recurrent tumour versus gliosis (10 cases) were included in this study. 201Tl single photon emission computed tomography (SPECT) was performed with the calculation of early and delayed uptake values and retention index. A high mean value of early and delayed uptake correlated with a low retention index in patients with high-grade astrocytoma in both postoperative residual and recurrent groups, versus a lower mean value of early and delayed uptake with a high retention index in patients with low-grade tumours. All postoperative cases with high-grade astrocytoma had high 201Tl uptake > 1.5, whereas 66.6% of cases with low-grade astrocytoma had low 201Tl uptake < 1.5. There was a correlation between retention index of 201Tl and tumour grade with r = 0.47. Also, recurrent cases showed remarkable differences in early and delayed 201Tl uptake (P < 0.05) and retention index (P < 0.001) compared with postradiation gliosis. There was a higher sensitivity in detection of tumour viability by 201Tl SPECT of 100% versus 80% using computed tomographic scanning and in the differentiation between recurrent tumour and postradiation gliosis.