Apolipoprotein AI (apoAI), a major component of high-density lipoproteins, is one of the major amyloid fibril proteins and a minor constituent of the senile plaques observed in Alzheimer's disease. We examined colocalization of apoAI in various kinds of systemic amyloidosis in this study. Forty-three of 48 formalin-fixed paraffin-embedded heart specimens with various forms of systemic amyloidosis reacted immunohistochemically with anti-human apoAI antibody. ApoAI was also detected in water-extracted amyloid material by immunoblotting. In addition, we observed colocalization of apoAI and murine amyloid A (AA) amyloidosis in human apoAI transgenic mice. This is the first report of colocalization of apoAI with amyloid deposits in various forms of human systemic amyloidosis and murine AA amyloidosis in human apoAI transgenic mice. ApoAI may not always be a major component of amyloid fibrils, even when it is present in systemic amyloid deposits.