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Since the 1930s, more than 130 inbred lines and 60 hybrid cultivars of onion have been released in the public sector in the United States. Other than breeder's reports from the period 1946-1965 and anecdotal information kept by onion workers, no systematic treatment of the pedigree of public onion germplasm releases has been developed. The objective of this research was to collect, characterize, and display the genetic relationships among more than 200 public onion germplasm sources used in the United States since 1931. Pedigree information revealed that most modern onion cultivars in the United States descend from a few open-pollinated populations brought to this country by immigrants. For example, selection in the open-pollinated populations Common Yellow and Silverskin by onion farmers in the eastern U.S. resulted in the formation of Yellow Globe Danvers, which was a precursor to virtually all Eastern storage onion germplasm in the U.S. Open-pollinated populations such Yellow Globe Danvers, Valencia, Sweet Spanish, Bermuda, and Grano formed the foundation germplasm for the first public U.S. onion breeding programs. Findings from this study suggest a relatively narrow germplasm base of public onion germplasm in the United States; however, this narrow pool coexists alongside significant gains through scientific breeding efforts, particularly during the past 75 years.