This article explores section 5(4) of the Mental Health Act, which permits nurses of a ‘prescribed class’ to detain an informal inpatient who is receiving treatment for mental disorder for up to six hours or until a doctor or approved clinician arrives. The article raises various issues in relation to the implementation of section 5(4), some, but not all of which are addressed by the relevant codes of practice for England and Wales. The authors raise important questions that nurses need to address if their holding power is to be applied appropriately in all cases.