A 55-year-old man with large B-cell lymphoma developed atraumatic left shoulder pain. 18F-FDG PET/CT revealed new left supraspinatus and infraspinatus muscle uptake while the initial disease resolved. Given the discrepancy between initial disease treatment response and new left shoulder findings, an MRI scan was performed. This demonstrated diffuse supraspinatus and infraspinatus muscle edema and enhancement with no focal lesion. Muscle biopsy was negative for lymphoma, but features of muscle denervation were seen. Overall, clinical and imaging findings were compatible with Parsonage-Turner syndrome (acute brachial neuritis), an uncommon condition that presented as a false-positive finding on PET/CT.