Endoplasmic reticulum aminopeptidase 1 (ERAP1) and the closely related ERAP2 are involved in the final trimming of peptides within the endoplasmic reticulum for presentation by major histocompatibility complex (MHC) class I molecules. ERAP1 was found to be associated with ankylosing spondylitis (AS) in a genome-wide association study of nonsynonymous single nucleotide polymorphisms, and this association has been confirmed in several studies. An ERAP1/ERAP2 haplotype has also been reported to be associated with familial AS. ERAP1 and ERAP2 could carry out several potential roles in the pathogenesis of AS. ERAP1-deficient mice show a considerable alteration in the level and repertoire of peptides presented by MHC class I molecules. Furthermore, ERAP1 has been shown to be involved in shedding cytokine receptors. Both of these functions require further analysis to better understand the exact role of ERAP1 in AS.