Our recent ultrastructural studies of amyloid angiopathy in biopsy specimens from Alzheimer's disease patients showed that perivascular cells and perivascular microglia are involved in the production of amyloid fibrils. Further examination of the walls of the vessels with and without amyloid deposits presented in this report reveals numerous mononuclear cells with a broad spectrum of morphological appearances. Some of these cells produce amyloid in the vascular wall and migrate into the neuropil. Others do not produce amyloid in this location but also migrate through the vascular basal lamina and position themselves on the external surface of basal lamina or in the neuropil outside the vascular astrocytic end-feet processes. The presence of clusters or rows of six or more of these cells in the position of perivascular microglial cells suggests their proliferation in the perivascular region. After leaving the perimeter of the vessel wall, perivascular cells become the perivascular, neuropil, and satellite microglia cells. Migrating perivascular cells become the microglia, which are engaged in amyloid fibril formation and development of classical and primitive plaques.