The Hopkins Competency Assessment Test (HCAT), a brief instrument for evaluating the competency of patients to give informed consent or write advance directives, consists of a short essay and a questionnaire for determining patients' understanding of the essay. In a study to validate the instrument, 41 medical and psychiatric inpatients answered the questionnaire after reading the essay while hearing it read aloud. A forensic psychiatrist who was blind to the HCAT scores later examined the patients for competency. A subject's number of correct answers to the HCAT questionnaire was an accurate indicator of clinical competency as assessed by the psychiatrist. The results suggest that the HCAT is a useful tool for rapidly screening patients for competency to make treatment decisions.