Background: Hospital readmission is a common, costly problem. Little is known regarding risk factors for readmission in older adults with cancer. This study aims to identify factors associated with 30-day readmission in a cohort of older medical oncology patients. Setting/Participants: Adults age 65 and over hospitalized to an Oncology Acute Care for Elders Unit at Barnes-Jewish Hospital. Measurements: Standard geriatric screening tests were administered in routine clinical care. Clinical data and 30-day readmission status were obtained through medical record review. Results: 677 patients met the inclusion criteria. 77% were white and 53% were male. Thoracic (32%), hematologic (20%), and gastrointestinal (18%) malignancies were most common. The 30-day unplanned readmission rate was 35.2%. Multivariable analyses identified complete dependence in feeding (odds ratio [OR], 3.70; 95% confidence interval [CI], 1.29-10.65), and some dependence (1.58, 1.04-2.41) and complete dependence (2.64, 1.70-4.12) in housekeeping, prior to admission, as associated with higher odds of readmission. Age. <. 75 (1.49, 1.04-2.14), African-American race (1.59, 1.06-2.39), potentially inappropriate medications (1.36, 0.94-1.99), and higher-risk reasons for index admission (1.93, 1.34-2.78) also increased odds of readmission. These factors were organized into a prognostic index. Conclusion: Hospital readmission was common and higher than previously reported rates in general medical populations. We identified several previously unrecognized factors associated with increased risk for readmission, including some geriatric assessment parameters, and developed a practical tool that can be used by clinicians to assess risk of 30-day readmission.