Ingestion of foreign anthropogenic material, here called junk, has been documented in many avian taxa but has become especially problematic in some Old World Gyps vulture populations within the last 30 years. Here, we document the effects of ingested junk on a reintroduced population of the Critically Endangered California Condor Gymnogyps californianus in southern California, U.S.A. Of 13 breeding attempts to date (2001–2005), only one has resulted in successful fledging. Of nests where either the nest substrate was sifted (n = 10) or nestlings (n = 8) were examined, all but one held junk. Nine nestlings hatched in the wild between 2002 and 2005; of these six died at or near nests and two were removed from the wild for health reasons. Four dead nestlings and two removed from the wild held substantial quantities of junk. In two cases, junk ingestion was determined to be the cause of death. Five of six dead nestlings had elevated hepatic copper levels (150–531 ppm dry weight) although the significance of this, if any, remains undetermined. In comparison with historic condor nests (n = 69), junk was more prevalent and of greater size and quantity in reintroduced condor nests. To date, junk ingestion has been the primary cause of nest failure in the reintroduced condor population and threatens the reestablishment of a viable breeding population in southern California.