The objective of this study was to assess the respective frequency of planned and unplanned early readmissions after discharge from an internal medicine department, and to identify and compare risk factors for these two types of readmissions. Readmissions within 31 days of discharge were identified as planned or unplanned based on analysis of discharge summaries. Time-failure methods were used to describe the risk of readmissions over time and to assess relationships between patient and index stay characteristics and risk of readmission. Of 5828 patients discharged alive, 730 (12.5%) were readmitted within 31 days. There were slightly more planned than unplanned readmissions (393 vs. 337). The difference in time-to-event functions was significant (P=0.04). The risk of planned readmission was increased for men, younger patients, and for patients discharged with a diagnosis of coronary heart disease, cardiac arrhythmia, and neoplastic disease. Increased risk of unplanned readmission was associated with index length of stay longer than 3 days, an increased number of comorbidities, and with a diagnosis of neoplastic disease. Planned readmissions constitute more than half of early readmissions to our internal medicine department. Therefore, a crude readmission rate is unlikely to be a useful indicator of quality of care. Several patient characteristics influence the risk of unplanned readmission, suggesting that case-mix adjustments are necessary when readmission rates are compared between institutions or tracked over time.