Rationale: Although several functional studies have demonstrated that positron emission tomography/computed tomography with 18 F-fluorodeoxyglucose (18 F-FDG PET/CT) appears to be efficient to identify a cerebral substrate in patients with known macrophagic myofasciitis (MMF), the predictive value of this imaging technique for MMF remains unclear. Patient concerns: We presented data and images of a 46-year-old woman. Diagnoses: The patient was referred to our center for suspected MMF due to diffuse arthromyalgias and cognitive disorder (involving an impairment of visual selective attention and a weakness in executive functions revealed by neuropsychological assessment) which occurred few years after last vaccine injections. Interventions: After a first negative deltoid muscle biopsy, a brain 18 F-FDG PET/CT was performed and revealed the known spatial pattern of a cerebral glucose hypometabolism involving occipital cortex, medial temporal areas, and cerebellum. Outcomes: Given the clinical suspicion of MMF and brain 18 F-FDG PET/CT findings, a 2nd deltoid muscle biopsy was performed and confirmed the diagnosis of MMF with typical histopathological features. Lessons: This case highlights the predictive value of brain 18 F-FDG PET/CT as a noninvasive imaging tool for MMF diagnosis, even when muscle biopsy result comes back negative. Abbreviations: 18 F-FDG = 18 F-fluorodeoxyglucose, MMF = macrophagic myofasciitis, PET/CT = positron emission tomography/ computed tomography.