Although foster care placement is often preceded by stressful events such as child abuse, foster care itself often exposes children to additional severe stressors. A history of foster care, as well as the childhood abuse that often precedes it, is common among homeless young adults. This study examined whether a history of foster care was associated with psychiatric disorders, prior psychiatric counseling, prescription of psychiatric medications, and prior psychiatric hospitalization among newly homeless young adults. A consecutive sample of 423 adults aged 18 to 21 years who sought emergency shelter for the first time between October 1, 2007, and February 29, 2008, were assessed at intake. Logistic regression analyses determined the associations between foster care and any psychiatric disorder (affective, anxiety, personality, and psychotic) and psychiatric treatment. The analyses adjusted for demographic characteristics, childhood abuse, substance use, prior arrest, unemployment, lack of high school diploma, and histories of psychiatric disorders and drug abuse among biological relatives. Homeless young adults with histories of foster care were 70% more likely than those without such histories to report any psychiatric disorder. They were more than twice as likely to have received mental health counseling for a psychiatric disorder, to have been prescribed psychiatric medication, and to have been hospitalized for psychiatric problems. Histories of foster care among homeless young adults should trigger screening for psychiatric disorders to aid in the provision of treatment (counseling, medication, and hospitalization) tailored to the psychiatric needs of this highly vulnerable population.