BACKGROUND On the eve of reform of the 1983 Mental Health Act (MHA), little is known about how decisions to admit people under its powers are made. OBJECTIVE To describe non-clinical and extra-legal influences on professionals' decisions about compulsory admission to psychiatric hospital. METHODS Participant-observation of MHA assessments, including informal and depth interviews with the practitioners involved, and follow-up interviews with the people who had been assessed. RESULTS A candidate patient's chance of being sectioned is likely to increase when there are no realistic alternatives to in-patient care. This typically occurs when staff have insufficient time to set such alternatives in place and are unsupported by other professionals in doing this. Outcomes may also be affected by local operational norms and the level of professional accountability for specific MHA decisions. CONCLUSIONS Non-clinical and extra-legal factors explain some of the geographical variation in MHA admission rates. If compulsion is to be used only in the 'last resort', administrators and policy makers should look beyond legislative change to matters of resource allocation and service organisation.