The histological location of amyloid within various organs in 25 cases of systemic AA amyloidosis was studied with a view to determine whether different morphological patterns exist in this category of amyloidosis. Although morphological variations due to progressive severity of disease were observed, there were appreciable variations in the patterns of amyloid deposition in the kidney and spleen that could not be simply explained on those grounds. Eleven (61%) of 18 kidneys examined showed severe glomerular involvement with mild degrees of vascular deposition while the remaining seven showed predominantly vascular involvement. The glomerular pattern appeared to be more ominous, being significantly associated with severe proteinuria or chronic renal failure. In nine (69%) of 13 spleens examined, amyloid was confined to the walls of small and medium-sized arteries while in the remaining four, vascular involvement was less severe and amyloid was deposited mainly along the reticulin of the white pulp. Possible explanations for these different patterns included resorption and redistribution of amyloid within the body during the course of the disease, and variation in tissue deposition as a manifestation of polymorphism of amyloid proteins. The latter appeared more feasible in view of the recent demonstration of SAA polymorphism and AA heterogeneity in man.