ASC was first identified as a caspase recruitment domain (CARD)-containing proapoptotic molecule that forms insoluble aggregates during apoptosis. Here, we report both the pyrin N-terminal homology domain (PYD) and CARD domains are involved in the aggregation of ASC. Preliminary experiments indicated that overexpression of ASC formed filament-like aggregates in COS-7 cells. Expression experiments using green fluorescent protein (GFP) constructs showed that not only the GFP-ASC-CARD but also the GFP-ASC-PYD formed filament-like aggregates in COS-7 cells. We confirmed these filament-like aggregates of both the ASC-PYD and the ASC-CARD due to homophilic interaction by immunoprecipitation method. We also demonstrated that the ASC-PYD associated with the ASC-CARD by heterophilic interaction. These observations suggest that the dimerization of the PYD as well as the CARD plays an important role in the oligomerization of ASC as an adaptor molecule.