Critics of the dangerousness standard for civil commitment contend that there is no professional standard for the evaluation of dangerousness. We used Three Ratings of Involuntary Admissibility, a reliable Index of behavioral indicators of danger to self, danger to others, and grave disability, and found that when combined into weighted patterns these indicators predicted disposition decisions of 70 clinicians in five psychiatric emergency rooms over 251 cases. A concurrent measure of perceived dangerousness, Clinician's Global Ratings of patients on these criteria, yielded similar results. We conclude that clinicians in California psychiatric emergency rooms apply a shared concept of dangerousness that can be described in behavioral terms.In the past three decades, involuntary confinement of the mentally ill has been dramatically curtailed. Several factors, including concern for the civil rights of patients, converged to bring about a decrease in reliance on institutional care and a narrowing of the grounds for civil commitment. Most state commitment statutes today specify that patients may be involuntarily admitted to a hospital only on grounds of danger to self, danger to others, or (in many states) grave disability, and then only if the condition results from mental disorder. Judicial interpretations of these statutes indicate that the common denominator, ie, the only legitimate basis for civil commitment, is danger due to mental disorder. [1][2][3][4] In the view of many commentators, 4,5 the adoption of the dangerousness standard constituted the most fundamental change brought about by commitment reform laws of the 1970s, laws modeled after California's Lanterman-Petris-Short (LPS) Act. 6 This standard