The purpose of this study was to corroborate experimental findings that long-circulating, superparamagnetic iron oxide contrast agents accumulate at the margins of human brain tumors, thereby improving their delineation on magnetic resonance (MR) images. This limited clinical study examined a total of four patients with brain tumors (three with primary gliomas and one with metastatic melanoma; n = 8 lesions) who were given a pharmaceutical formulation of a superparamagnetic, ultra-small-particulate iron oxide (USPIO, intravenous dose of 1.1 mg Fe/kg). The agent has a characteristically long plasma half-life and is currently undergoing Phase III clinical trials for liver disease (AMI-227, Advanced Magnetics, Cambridge, MA). MR (conventional spin-echo and gradient-echo) images of the brain were obtained before and 12, 24, and/or 36 hours after administration of the agent, with follow-up several weeks later. Twelve to 36 hours after IV administration of the USPIO, both primary and metastatic brain tumors showed readily detectable increases in signal intensity on T1-weighted spin-echo images. Unlike the pattern of enhancement with a gadolinium (Gd) chelate, which occurred immediately and decreased within hours, that with the USPIO occurred gradually, with a peak at 24 hours, and decreased over several days. Whereas the enhancing tumor margin with the Gd chelate blurred with time due to diffusion of the agent, the margin with the USPIO remained sharp, presumably due to the much lower diffusion coefficient (large size) of the particles and partly because of local endocytosis by tumor cells. Compared with Gd chelates, long-circulating, superparamagnetic iron oxide contrast agents can provide prolonged delineation of the margins of human brain tumors on MR images, which has implications for the targeting of diagnostic biopsies and the planning of surgical resections.