The results of well-characterized two-site enzyme immunoassays showed that the crude leptomeninges (consisting of the pia matter, arachnoid matter, and leptomeningeal vessels [LV]) from aged control brains and brains affected by Alzheimer disease (AD) contain very high levels of amyloid β-protein (Aβ). To learn about the source of Aβ, we carefully dissected out both leptomeninges (LM) and LV under a dissecting microscope and determined the levels of soluble Aβ in each. The purity of these dissected tissues was confirmed by the absence or presence of α-smooth muscle actin representing LV by Western blotting. Surprisingly, the amounts of Aβ in each dissected sample were nearly equivalent on a weight basis. In each compartment from aged controls the level of Aβ1–42 was comparable to that of Aβ1–40, while in AD brain Aβ1–40 was a predominant species in both LM and LV. In some cases careful immunocytochemical examination revealed the presence of Aβ deposits that were immunolabeled by several Aβ monoclonal antibodies in leptomeningeal layers (most often in the arachnoid matter). The extent of Aβ deposition in LM appeared to be much less than that explained by the soluble Aβ levels, suggesting that immunocytochemically undetectable Aβ can accumulate in LM. These observations indicate that leptomeninges are a large reservoir of Aβ in normal aged individuals and in AD patients.