Objectives: Mental health laws set criteria that limit the use of involuntary admission to specific circumstances, and clinicians are expected to justify the lawfulness of such detention by referral to these criteria. The South Australian Mental Health Act 1993 required grounds to be documented on the detention form, specifically with respect to the presence of a mental illness, risk to self or others, and a need for immediate treatment. This investigation sought to determine whether the grounds provided for detention met legislative requirements. Design and setting: 2491 consecutive forms authorising the initial detention of involuntary patients in South Australia between July 2008 and June 2009 were rated to determine whether criteria stipulated by legislation were addressed. Results: Only 985 forms (40%) addressed all the legal requirements for detention. 1471 forms (59%) did not comment on a requirement for immediate treatment, 540 (22%) did not state the presence of mental illness, and 359 (14%) did not discuss risk to self or others. Given the particularly poor performance of clinicians in addressing the need for immediate treatment, the data was reanalysed with respect to the presence of mental illness and risk only; 1697 forms (68%) addressed both these criteria. Conclusions: This low compliance rate with legal requirements is concerning. It may reflect clinical decision making, the attention given to the form by the physician filling it, or a combination of both. Stating the grounds for involuntary admission should provide protection for the rights of patients, and the requirement to do so reflects the gravity that the loss of liberty entails for the patient. Our findings are relevant to jurisdictions that are currently reviewing mental health legislation and the need to document the grounds for involuntary treatment, including South Australia.