The main aim of this study was to determine whether child mental health professionals who regularly use questionnaires to screen for mental disorders know that these questionnaires have a low predictive value when the base rate of a disorder is low. The study was based on a representative sample of professionals who used questionnaires regularly to screen for mental disorders. They were set a problem in which a clinic sample was screened with a questionnaire that, at a certain cut-point, had 80% sensitivity and 80% specificity, and in which the true base rate of disorder was 10%. Only 10% (5/48) of respondents answered correctly that just 30% of individuals who scored above this cut-point would actually have the disorder and more than half of respondents believed that 80% would have the disorder. Both users and designers of questionnaires need to be more aware of and explicit about their drawbacks as screens for mental disorders.