Spinal arachnoiditis is a disease characterized by inflammation and scarring of the arachnoid membrane of the spinal cord. FDG PET/CT findings of spinal arachnoiditis have been rarely reported. We present 3 cases of spinal infectious arachnoiditis (1 pyogenic and 2 tuberculous). MRI was performed in these 3 cases showing thickened spinal meninges with marked enhancement. FDG PET/CT was performed in 2 cases showing intense linear FDG uptake along the spinal canal. The combination of MRI and FDG PET/CT may be helpful in identifying the infectious sites and evaluating the activity of the infection.