Background: Emergency room (ER) visits during chemotherapy are costly. To identify potential causes of ER visits, we interviewed oncology patients and providers about symptoms and other factors related to ER visits. Methods: People with cancer (n = 19) and oncology physicians and nurses (n = 11) were interviewed using a semi-structured protocol. Interviews were transcribed. The study team developed a codebook and interviews were coded deductively. Four of the interviews were double coded. Results: Patients reported communicating with their physicians mostly by telephone. Patients reported a variety of distressing symptoms from chemotherapy including infection, mouth sores, distress, fatigue, neuropathy, gastrointestinal (GI) effects. Pain, fever, neuropathy, and dizziness were symptoms that motivated ER visits. Patients reported that contact with their care team and having a specific plan for symptoms helped prevent ER visits. Patients also reported that going through chemotherapy the first time was harder because they did not know what to expect and the patient education can be overwhelming. Providers did not always have formal protocols for proactively contacting patients but those that did contacted patients early in treatment. Providers reported GI symptoms, pain, shortness of breath, dizziness, and infection as causes of ER visits. They reported difficulties assessing symptoms due to reporting inaccuracies and reticence to share details. Providers cited financial insecurity, remote living, and lack of social support as factors increasing risk of ER visits. Providers reported some patients would not call frequently enough while a smaller subset called more frequently than providers perceived as needed. Conclusions: Results suggest contact with the care team is crucial for preventing ER visits during chemotherapy. Symptoms causing ER visits were consistent with previous literature