Abstract During atherosclerosis, cholesterol precipitates into cholesterol crystals (CC) in the vessel wall which trigger plaque inflammation. In the present, study we examined the relation between complement, NLRP3 signalling pathways and CC in patients with coronary artery disease and carotid atherosclerosis. We analysed plasma, peripheral blood mononuclear cells (PBMC) and carotid plaques from patients with different degrees of atherosclerosis applying ELISAs, multiplex, qPCR, immunohistochemistry, gene profiling and bioinformatics. Transcripts of IL-1β and NLRP3 were increased and correlated in PBMC from patients with acute coronary syndrome. Immunohistochemistry of carotid plaques showed pronounced accumulation of C1q and terminal C5b-9 complement complex (TCC) around CC-clefts. Transcripts of C5aR1, C5aR2 and C3aR1 were increased in carotid plaques compared to control arteries. Priming of carotid plaques ex vivo with C5a and TNF followed by CC incubation resulted in pronounced upregulation of NLRP3 inflammasome components. Importantly, we found that intracellular “C5 system” is a major contributor of CC induced IL-1β in human macrophages through the action of C5aR1 present on mitochondria. Mechanisms underlaying the role of intracellular C5 in this process will be described. The data suggest that CC formation is an important extracellular and intracellular complement activator which in turn results in IL-1β expression and activation of NLRP3 in atherosclerotic plaques.