Preventing readmissions after hospital discharge is a national policy priority. The Partnership for Patients has established a goal of reducing preventable readmissions by 20% by 2013, and hospitals now face financial penalties for excess readmission rates. However, the proportion of readmissions that is truly preventable remains unclear, as prior studies have found that only 1 in 5 readmissions may be preventable. This case series from the integrated Kaiser Permanente system found that nearly half of their 30-day readmissions were at least possibly preventable (with 11% being completely preventable). Most readmissions had multiple contributing causes, and interestingly, use of strategies to prevent readmissions such as postdischarge telephone calls and early primary care follow-up appointments varied widely across the 18 hospitals in the study. A potentially preventable readmission due to a medication error is discussed in an AHRQ WebM&M commentary.