Early and delayed 201Tl SPECT studies were performed on 13 patients with intracranial meningiomas, which were classified in three groups according to their histological types: meningothelial (n = 7); transitional and fibroblastic (n = 3); and malignant types (n = 3). The early uptake indices (UI, ratios of average counts/pixel in the lesion to those of the contralateral area on early images) were relatively high in all types: meningothelial meningiomas, 5.75 +/- 2.16 (mean +/- s.d.); transitional and fibroblastic meningiomas, 4.69 +/- 0.54; and malignant meningiomas, 7.10 +/- 3.72. There were no statistical differences in relation to histological type. The delayed uptake indices were 2.65 +/- 0.89, 3.37 +/- 1.02, and 5.16 +/- 1.62, respectively. Statistically, the delayed UI of meningothelial meningiomas were lower than those of malignant types (p < 0.05). The retention indices (RI, ratios of delayed to early UI) were 0.48 +/- 0.08, 0.79 +/- 0.16, and 0.84 +/- 0.16, respectively. The RI of the meningothelial type were also statistically lower than those of the other two groups (p < 0.05). There were no statistically significant differences between transitional plus fibroblastic types and malignant meningiomas. We conclude that 201Tl exhibits early high accumulations in all types of meningiomas, but its retention rates probably differ according to histological types, and a high retention index is predictive of the malignant potential in a meningioma.