We describe a patient in remission from acute lymphoblastic leukemia who developed a painless common peroneal neuropathy. Magnetic resonance imaging (MRI) revealed nerve thickening and enhancement, while a positron emission tomography (PET) scan demonstrated increased fluorodeoxyglucose uptake in a large segment of the neurovascular bundle, suggesting peripheral nerve infiltration. Both findings resolved following treatment with chemotherapy that crossed the blood-nerve barrier. In selected patients presenting with peripheral neuropathy, MRI and PET scan can be helpful in the diagnosis of peripheral nerve infiltration.