Objectives: This study describes the risk factors affecting the unexpected readmission of 261 patients who were discharged from a university hospital in Seoul. Methods: This case-control study reviewed medical records of inpatients who had been discharged from a hospital between 1 August 1995 and 31 October 1995 after the treatment for general diseases. The cases were 68 patients who were readmitted unexpectedly within 28 days of discharge from an index stay, and the controls were 193 Patients who were discharged without readmission during the study period. Results: Logistic regression analysis results were as follows; Patients who had no operation during their hospital stay were more likely to be readmitted unexpectedly than patients who had operation. Patients who had 1 or 2 parts of their body being involved in treatment were more likely to be readmitted unexpectedly than patients who hand more than 3 parts of their body being involved in treatment. Patients who had complications after surgery were more likely to be readmitted unexpectedly than patients who had no complications. Insufficient discharge planning caused unexpected readmissions. Conclusions: Discharge planning education should be extended to health care providers. And the assessment of discharge planning should be evaluated. Readmission is often necessary for the treatment of related problems of originating from initial hospitalization, which causes cost problems. Unexpected readmission is preventable and the models for readmission can serve as a valuable clinical tool for high risk patients.