Studies of the prevalence, natural history, and outcome of unrecognized mental illness in general medical outpatient settings were reviewed. Approximately half of the patients with a psychiatric disorder were not recognized as having a mental illness by their primary care physician. The natural history of unrecognized mental illness suggests a poorer clinical course of anxiety disorders but not for depressive disorders. Most interventions demonstrated a significant improvement in the physician's ability to identify and treat psychiatric disorders but had a minimal effect on patient symptoms and no effect on short-term health care use. Although primary care physicians do not recognize psychiatric disorders in a high percentage of patients, efforts to improve recognition may not lead to decreased patient suffering or decreased health care costs.