Tumor-associated macrophages (TAMs) have been linked to tumor development and progression. TAMs enhance proliferation of tumors through promoting growth but also survival of the tumor by preventing attack by natural killer and T cells. There is a growing interest in targeting TAMs in oncology therapeutic development, given their noted protumoral activity. Ferumoxytol has been shown to be taken up by TAMs and therefore is potentially a good imaging agent to track TAM migration into and out of tumors. Ferumoxytol (FMX), an ultra-small carbohydrate coated iron-oxide containing nanoparticle was developed to treat anemia in patients with chronic renal failure. One of the properties of this agent is its prolonged residence time within the intravascular space due to an average particle size of approximately 30 nm and slow clearance from the blood stream. When performing Magnetic Resonance Imaging (MRI) studies in subjects given this agent for off-label use for the treatment of their anemia, exquisite angiographic images can be obtained out to 24 hours post injection. The enhancement of the intravascular space following FMX administration has provided a novel approach to imaging pathologic conditions that involve the vascular tree such as in stroke, vascular malformations, chronic renal disease and tumor vascularity, even in patients without kidney disease. Here we discuss the application of FMX in clinical trials.