Genetic analysis of onion has been hampered by a lack of portable co-dominant markers based on the polymerase chain reaction (PCR). The public release of a relatively large set of non-redundant onion expressed sequence tags (ESTs) in 2003 has provided the opportunity to develop such markers for use in Allium research and industry. We have mined this collection for simple sequence repeats (SSRs) and screened over 200 primer sets designed to flank SSRs for their ability to amplify low-copy polymorphic products in Allium cepa and A. roylei. Screening to date confirms that these are a rich source of polymorphic markers in Allium. Half of the sets reveal polymorphism between onion and A. roylei, ~40% reveal putative allelic variation in a small set of onion germplasm and 20% reveal polymorphism in a single F2 family in the cross `W202A¿ x `Texas Grano 438¿. Mapping common markers has permitted alignment with the RFLP map developed in the `Brigham Yellow Globe 15-23¿ x `Alisa Craig 43¿ population. Preliminary studies show these markers are effective in distinguishing variation in allelic diversity among a set of inbred and open-pollinated onion varieties. Our results suggest that these markers will enable effective genome scanning and analysis of genetic diversity and identity in cultivated onion.