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A 55-year-old man presented with recurrent brain infarction which had increased multifocally mainly in the cerebral white matter over the course of one year. Antibodies associated with antiphospholipid syndrome (APS) were initially negative. The patient was admitted to our department because of the thickened meninges shown on gadolinium enhanced brain MRI, mimicking hypertrophic pachymeningitis. However, blood and cerebrospinal fluid analysis revealed no significant inflammatory changes. On histopathological examination of the biopsied meninges, the arachnoid membrane was thickened with fibrosis, and arachnoidal microvessels were enlarged without significant inflammatory changes. The dura mater was not thickened, and no inflammation or microvessel enlargement were revealed. Finally, serum IgG anticardiolipin antibody testing was positive twice at an interval of more than 12 weeks, confirming the diagnosis of APS. Since initiating antithrombotic therapy with warfarin, brain infarction has not recurred. Without inflammation in the arachnoid membrane, the congestion of blood flow caused by thrombosis of microvessels in the arachnoid membrane might have increased the thickness of the arachnoid membrane.