Chinook salmon (Oncorhynchus tshawytscha), hatchery-reared as juveniles, returned to the upper Columbia River Basin in numbers exceeding broodstock and fishery needs during the spring of 2000. Plans to euthanize these adults were opposed by some regional stakeholders, who preferred letting them spawn naturally in streams also used by endangered spring-run chinook salmon. The National Marine Fisheries Service requested that the Independent Scientific Advisory Board review the scientific literature and conclude whether it was biologically sound to permit hatchery-origin adult salmon to spawn in the wild in large numbers. Substantial experimental evidence demonstrates that domestication selection can genetically alter hatchery populations in a few generations and that hatchery-origin adults returning from the ocean and spawning in the wild produce fewer progeny than adults of wild origin spawning in the wild. More limited evidence suggests that interbreeding between hatchery-origin adults and wild fish can reduce the fitness of the wild population. We conclude that decisions whether or not to permit hatchery-origin adults to spawn in the wild should be based on the needs of wild populations and the ability of the habitat to support additional reproduction, not based simply on the availability of hatchery-origin adults returning from the ocean.