The purpose of this study is to characterize the time course of N-isopropyl-p-[123I]-iodoamphetamine (123I-IMP) uptake of primary central nervous system lymphomas (PCNSL) comparing with those of other common brain tumors such as malignant gliomas and meningioma. Ten patients with PCNSL, 14 with malignant gliomas and 7 with meningioma underwent 123I-IMP single photon emission computerized tomography (SPECT). Four (40%), 7 (70%) and 9 cases (90%) of PCNSL showed IMP index more than 1 on early (15 min), delayed (6 h) and extra-delayed (24 h) SPECT images, respectively. In contrast with those IMP index was less than 1 on any image of both malignant gliomas and meningioma. The PCNSL cases with IMP index more than 1 could be distinguished from malignant gliomas and meningioma visually. The mean value of IMP index of PCNSL was increased time-dependently and was higher than those of the other two brain tumor groups on SPECT images of any time phases, and statistically different from those of malignant gliomas, however, not from those of meningioma. Interestingly, the retention rates of 123I-IMP from early to extra-delayed image and delayed to extra-delayed image in PCNSL were significantly higher than those of both malignant gliomas and meningioma. The high 123I-IMP retention on SPECT image is regarded as characteristic in PCNSL and the evaluation of this character is suggested to be more useful for diagnosis of this tumor than that of 123I-IMP up-take on a single SPECT image.