Unplanned hospital admissions (UHA) in older adult populations are a recurring problem in older adults with cancer. Older adults comprise approximately 60% of cancer diagnoses and receive the majority of cancer treatment. However, little is known about why older adults under treatment for cancer experience a high number of unplanned hospital admissions. A review of the literature provided few study findings and a gap in the current knowledge was identified regarding the factors associated with unplanned hospital admissions in older adults under treatment for cancer. A conceptual framework based on the literature and this researcher's clinical experienced guided this study. The purpose of this study was to explore the factors related to unplanned hospital admissions and determine if one or more factors are predictive of unplanned hospital admissions of older adults with cancer. A convenience sample of 129 dyads of older adults with cancer and their family caregivers were approached and enrolled in the adult oncology outpatient infusion centers and inpatient units within a community cancer center in central Florida. Patient demographic and clinical data were obtained through a retrospective medical record review. Family caregiver demographic and side effect knowledge data was collected prospectively during interviews with family caregivers using a newly developed tool, Nurse Assessment of Family Caregiver Knowledge and Action Tool (NAFCKAT). The NAFCKAT contains 11 items to determine baseline knowledge about side effects and plan for managing side effects. A fever subsection consists of 4 knowledge and 2 action questions and a dehydration subsection consists of 2 knowledge and 2 action questions. Preliminary research was conducted to determine reliability and validity of the NAFCKAT. Excellent inter-reliability was found for the tool and preliminary support for validity was determined for the fever subscale. Descriptive statistics and logistic regression analyses were used to evaluate data collected from patient medical records and NAFCKAT scores. Study findings revealed that unplanned hospital admissions were more likely to occur when older adults had the presence of impaired function prior to treatment initiation and/or experienced side effects of infection /fever and vomiting/diarrhea during treatment. The presence of impaired function and family caregiver support (knowledge and availability) did not moderate the relationship between side effects and unplanned hospital admissions. Findings suggest that the presence of impaired function and side effects of infection and fever, and vomiting and diarrhea, predict unplanned hospital admissions in older adults during the active cancer treatment phase. Nurses should advocate for and conduct targeted assessments to identify the presence of functional impairments prior to cancer treatment initiation. In addition, nurses should actively monitor for the presence of cancer treatment-related side effects during the treatment phase of the cancer trajectory. Information gained from these assessments will assist nurses to provide practical and tailored strategies to support older adults and their family caregivers during cancer treatment and reduce the risk for unplanned hospital admissions.