Acts of self-poisoning and self-injury carried out by in-patients and day patients while in treatment at two psychiatric hospitals were recorded over a one-year period. Comparison with results from a survey based on referrals to a general hospital in the same area demonstrated that the rate at which these events occurred was over 50 times greater for patients in psychiatric care than in the general population. A larger proportion of the psychiatric hospital incidents involved self-injury. Analysis of the records of a consecutive series of all psychiatric hospital patients revealed that at least one-third had a previous history of self-poisoning or self-injury. Of the patients carrying out such acts in one hospital, 77 per cent had a history of this behaviour prior to admission. Clusters of incidents were identified. Possible factors underlying this behaviour in the hospital setting and means of prevention are suggested.