A 53-year-old man suffered from pulsating headache for 2 months. Magnetic resonance imaging with gadolinium revealed a linear or nodular mass along the left fronto-parietal convexity. Positron emission tomography (PET) with L-[methyl-11C]methionine (11C-MET) demonstrated increased uptake in the enhanced lesion. Biopsy, obtained by craniotomy, demonstrated granulation with lymphocyte and plasma cell infiltration, suggesting inflammatory changes, and a diagnosis of idiopathic hypertrophic cranial pachymeningitis (IHCP) was made. Steroid therapy resulted in improvement of the clinical symptoms and shrinkage of the enhanced lesion in a week. Follow-up 11C-MET PET study, after 18 months of steroid therapy, demonstrated significantly decreased uptake in the lesion, so the steroid therapy was discontinued. Neither clinical nor radiological recurrence was observed one year after discontinuation of the steroid therapy. This case of IHCP with increased 11C-MET uptake, which then decreased after steroid therapy suggests that 11C-MET PET is a useful monitoring modality for therapeutic efficacy against IHCP, and can indicate the appropriate timing of therapy discontinuation.