Of the 622 patients admitted to a geriatric medical unit during the period November 1985 to October 1986, 211 (33.9%) had been discharged from hospital within the previous year. Of these, 117 (55.5%) had previously been admitted to the geriatric medical unit (18.8% of all admissions). The reasons for readmission were deterioration of existing disease (47.4%), new medical events (33.2%), poor management of previous discharge (9.5%), and social problems (5.7%). Age was not a significant factor influencing readmission. Readmission was judged preventable by better management of the previous admission and discharge in only 33 cases (15.6% of readmissions). A prospective study of the patients discharged in the same year revealed that 19.3% were readmitted in the following year, and of these readmissions 2.3% were considered to be preventable. There was no difference in medical causes for readmission in the two years. A significant number of the patients readmitted in both years studied was categorized as "high dependency". The figures suggest that although unplanned readmission is common, it is generally not preventable by better management of the previous admission, and often reflects a highly dependent and medically frail group of patients, in whom early intervention with readmission is appropriate.