A random sample of 133 elderly patients who had an unplanned readmission to a district general hospital within 28 days of discharge from hospital was studied and compared with a matched control sample of patients who were not readmitted. The total group was drawn from all specialties in the hospital, and by interviewing the patients, their carers, the ward sisters, and the patients' general practitioners the factors causing early unplanned readmission for each patient were identified. Seven possible principal reasons were found: relapse of original condition, development of a new problem, carer problems, complications of the initial illness, need for terminal care, problems with medication, and problems with services. There were also contributory reasons, and it was usual for several of these to be present in each case. The unplanned readmission rate was 6%; the planned readmission rate was 3%. It was thought that unplanned readmission was avoidable for 78 (59%) patients. Patients in the study group and in the control group showed significant differences in certain characteristics--such as low income, previous hospital admission, already having nursing care, and admission by general practitioners--and this might help to identify patients who are likely to be readmitted in an emergency.