Three experiments assessed how revealing hidden a stigmatizing condition to others can affect task performance. In Experiment 1, participants were undergraduates who either had or had not previously received treatment for a mental illness. All students completed a test based on the Analytic portion of the GRE. Before starting the test, however, half of the students were asked questions designed to reveal their mental health history and were told that the test was diagnostic of their ability (stereotype threat for individuals with a history of mental illness) or did not answer such questions and were told that the test was not diagnostic of ability (control). The poorest test performance was exhibited by individuals with a history of mental illness who were prompted to reveal this history. In Experiment 2, female students who did or did not have a history of depression were either prompted to reveal or did not have to reveal this medical history. After doing so, all participants completed two tests, one of which supposedly was diagnostic of ability and one which was not. As in Experiment 1, having to reveal a history of mental illness reduced performance on the diagnostic test. Performance on the supposed nondiagnostic test was also negatively affected, though to a lesser degree. Experiment 3 focused on whether revealing any stigmatized condition would harm performance. Female undergraduates with either a history of mental illness, an eating disorder, or no history of either mental illness or eating disorder were either prompted to reveal their history or not before completing an intellectual test. Only women with a prior history of mental illness performed more poorly on the test after revealing their history. These results show that revealing a stigma associated with mental incompetence can undermine task performance.